Method and apparatus for reducing a nitrogen oxide, and control thereof

ABSTRACT

Disclosed herein is a method and apparatus for reducing a nitrogen oxide, and the control thereof.

This application claims the benefit of U.S. Provisional Application No.60/389,781, filed on Jun. 19, 2002, which is incorporated in itsentirety as a part hereof for all purposes.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for reducing a nitrogenoxide. In particular, it relates to the use of a gas analyzer to obtaininformation related to the compositional content of a multi-componentgas mixture that contains a nitrogen oxide for the purpose of assistingin the control of the reduction.

TECHNICAL BACKGROUND

Oxides of nitrogen (NO_(X)) that are emitted by an emissions source,such as those formed as a result of combustion, are included among themain causes of the “acid rain” problem, the photochemical smog problemand the resulting damage to the environment. These harmful substancesshould therefore be eliminated to the greatest extent possible from thegases emitted by an emissions source, such as the exhaust from acombustion process, prior to their discharge into the atmosphere.

One source of nitrogen oxides, in the form of NO₂ and mainly NO, arethose formed by the combustion of coal, oil, gas, gasoline, diesel fuelor other fossil fuels. Combustion of fossil fuels occurs, for example,in a stationary device such as furnace, which is a device for theproduction or application of heat. A furnace may be used in connectionwith a boiler such as in a steam generator that drives a steam turbinein an electrical generating plant, in connection with an industrialoperation such as in a smelter or chemical reactor, or in connectionwith supplying heat for human consumption.

Fossil fuels are also combusted in a mobile device, including a devicethat supplies mechanical power such as an internal combustion engine ina vehicle for transportation or recreation, or in a piece of equipmentfor construction, maintenance or industrial operations; or in a gasturbine, which is a turbine driven by a compressed, combusted fluid(such as air), such as in the engine of a jet aircraft. Gas-emittingdevices such as an internal combustion engine or a gas turbine are alsofound in stationary applications, however. The exhaust gas emitted bydevices such as those described above is a multi-component mixture ofgases containing nitrogen oxides. Nitrogen oxides are also emitted byplants for the incineration of industrial or municipal waste. Inaddition, carbon monoxide and hydrocarbons are also emitted by thesesources.

A problem exists with respect to the need for control of the injectionof a reducing agent into a gas mixture containing nitrogen oxides. Thereis a desire to effect the reduction of as large a quantity of thenitrogen oxides present in the gas mixture as possible. For thispurpose, what amounts to a stoichiometric excess of reducing agent, interms of the quantity of nitrogen oxides present, is often injected intothe gas mixture and thus into the nitrogen oxides. An excess of reducingagent is employed not so much by design but primarily because of theunavailability of information related to the compositional content ofthe gas mixture sufficient to accurately calculate the stoichiometricequivalent of reducing agent needed. The compositional content of a gasmixture containing nitrogen oxides often varies in an extremelyunpredictable manner as it moves through a conduit from its emissionsource to the point of its ultimate destination, such as a point ofdischarge into the atmosphere. As a result, because of the desire toobtain reduction of a large percentage of the nitrogen oxides, an amountof reducing agent is injected that later proves to be an excess. Whetherthis results from calculations based on inaccurate or incompleteinformation, a strategy of employing an excess to be certain that toolittle is not employed, or incomplete reaction of whatever the amount,the same undesired consequence is experienced—unreacted reducing agentis discharged to the atmosphere and becomes a pollutant itself. Whenammonia is the reducing agent, this is known as ammonia slip. In a gasmixture that is unscrubbed, or otherwise contains sulfur oxides,unreacted ammonia is also capable of reacting with the sulfur oxides toyield corrosive, sticky deposits of ammonium sulfate and/or ammoniumhydrogen sulfate that foul the mechanism of the conduit.

There is a need then for a method and apparatus for the reduction of anitrogen oxide that provides control of the reaction of reduction, andin particular control of the injection of a reducing agent into the gasmixture containing the nitrogen oxide. In particular, there is a needfor a method and apparatus that enables the calculation of the amount ofreducing agent to be injected in relation to information about thecompositional content of the gas mixture.

This invention addresses those needs by providing a method and apparatusin which analysis of the gas mixture is performed to furnish informationrelated to the compositional content thereof. In certain embodiments,the analysis is furnished by a gas analyzer that may be placed within aconduit through which the gas mixture is transported in positions thatcreate an opportunity to develop useful information about the gasmixture, and especially information related to the nitrogen oxidecontent thereof. In certain other embodiments, a gas analyzer isemployed for this purpose that outputs a signal related to the contentwithin the gas mixture of an individual component gas therein and/or thecollective content of a sub-group of gases therein. In certain otherembodiments, the information is inputted into a decision making routineand/or a map, and may be used to calculate a desired amount of reducingagent to be injected into the gas mixture, and thus into the nitrogenoxides to be reduced. Other embodiments of the invention are as moreparticularly described below, or are as would be apparent to the artisanin view of the description below.

SUMMARY OF THE INVENTION

One embodiment of this invention is an apparatus for reducing a nitrogenoxide gas emitted by a emissions source that involves (a) an exhaustconduit for transporting the nitrogen oxide gas downstream from theemissions source, (b) an injector for injecting a reducing agent intothe conduit, and (c) one or more gas analyzers located in the conduitupstream from the injector.

Another embodiment of this invention, in a multi-component gas mixturethat is emitted by a emissions source and contains a nitrogen oxide,wherein a nitrogen oxide is reduced by injecting a reducing agent intothe gas mixture, is a method of determining the amount of reducing agentto be injected, or of decreasing the amount or release of unreactedreducing agent, by determining information as to the compositionalcontent of the gas mixture, and controlling the injection of thereducing agent in relation to the information as to the compositionalcontent of the gas mixture.

Another embodiment of this invention, in a multi-component gas mixturethat is emitted by a emissions source and contains a nitrogen oxide,wherein a nitrogen oxide is reduced by injecting a reducing agent intothe gas mixture and contacting the gas mixture with a catalyst, is amethod of determining the amount of reducing agent to be injected, or ofdecreasing the amount or release of unreacted reducing agent bydetermining information as to the compositional content of the gasmixture before the gas mixture contacts any catalyst, and controllingthe injection of the reducing agent in relation to the information as tothe compositional content of the gas mixture.

Another embodiment of this invention, in a multi-component gas mixturethat is emitted by a emissions source and contains a nitrogen oxide,wherein a nitrogen oxide is reduced by injecting a reducing agent intothe gas mixture and contacting the gas mixture with a catalyst, is amethod of determining the amount of reducing agent to be injected, or ofdecreasing the amount or release of unreacted reducing agent bydetermining information as to the compositional content of the gasmixture after the gas mixture has contacted a catalyst, and controllingthe injection of the reducing agent in relation to the information as tothe compositional content of the gas mixture.

Another embodiment of this invention, in a multi-component gas mixturethat is emitted by a emissions source and contains a nitrogen oxide,wherein a nitrogen oxide is reduced by injecting a reducing agent intothe gas mixture and contacting the gas mixture with a catalyst, is amethod of determining the amount of reducing agent to be injected, or ofdecreasing the amount or release of unreacted reducing agent, bydetermining information as to the compositional content of the gasmixture after the gas mixture has contacted all catalyst, andcontrolling the injection of the reducing agent in relation to theinformation as to the compositional content of the gas mixture.

DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an array of chemo/electro-active materials.

FIG. 2 is a schematic of the pattern of interdigitated electrodesoverlaid with a dielectric overlayer, forming sixteen blank wells, in anarray of chemo/electro-active materials.

FIG. 3 depicts the electrode pattern, dielectric pattern, and sensormaterial pattern in an array of chemo/electro-active materials.

FIG. 4 is a schematic layout of the flow of a gas, such as thecombustion exhaust from a boiler, through an SCR system.

FIG. 5 is a schematic layout of the flow of a gas, such as thecombustion exhaust from a boiler, through an SCR system.

FIG. 6 shows the placement of a catalyst or a catalyst bed in an SCRsystem.

FIG. 7 is a schematic layout of the flow of a gas, such as thecombustion exhaust from a boiler, through an SCR system containing a gasanalyzer.

FIG. 8 is a schematic diagram of an internal combustion engine showingthe placement of a gas analyzer.

FIG. 9 is a schematic diagram of an internal combustion engine showingthe placement of a gas analyzer in connection with an SCR system.

DETAILED DESCRIPTION OF THE INVENTION

Nitrogen oxides may be reduced by contact with a reducing agent in theabsence of a catalyst at a temperature of about 850 to about 1200° C.,preferably about 900 to about 1100° C. This is usually referred to asselective non-catalytic reduction. The most common way of providing atemperature high enough to perform the reduction is to inject thereducing agent into the gas mixture that contains the nitrogen oxides inor near the source, such as a source of combustion, from which thenitrogen oxides are being emitted. The nitrogen oxides are predominantlytransformed by the high temperature of the source of emissions tomolecular nitrogen, which is nontoxic. Ammonia (e.g. anhydrous ammonia)is a reducing agent typically used, but urea is an alternative choice asa reducing agent. Three to four times as much reducing agent is requiredin a non-catalytic reduction, as compared to a catalytic reduction(described below), to achieve the same extent of reduction.

More common, then, is selective catalytic reduction, in which diminutionof the nitrogen oxide emitted by an emissions source, such as a sourceof combustion, takes place through contact of the nitrogen oxide and thereducing agent with a catalyst. In order to ensure an optimalutilization of the needed reducing agent, selective catalytic reductionprocesses are preferred for the removal of nitrogen oxides fromemissions sources such as a combustion exhaust because of the oxygencontent in the exhaust gas. As a reducing agent, ammonia gas (e.g.anhydrous ammonia) has proven itself to be suitable because it reactseasily with oxides of nitrogen in the presence of an appropriatecatalyst for the reaction, but only to a slight extent with the oxygenpresent in the gas. Urea is an alternative choice as a reducing agent.

For the selective reduction of the nitrogen oxides contained incombustion exhaust gases, for example, it is known to feed into theexhaust gas stream vaporous ammonia (NH₃) under pressure, or ammoniadissolved in water, without pressure, while an effort is made, by meansof a mixing section with appropriate baffling within the adjoiningconduit gas passages, to achieve a streamer-free distribution of ammoniaand temperatures in the flow of exhaust gas. The gas mixture emittedfrom a furnace flue may contain, for example, 1-20 percent by volume O₂,40 to 2000 ppm by volume nitrogen oxides, and 10 to 5000 ppm by volumeSO₂ and SO₃. The catalytic reduction of the nitrogen oxides by use ofammonia as a reducing agent typically proceeds according to one or moreof these reactions:

4NO+4NH₃+O₂→4N₂+6H₂O   I

2NO₂+4NH₃+O₂→3N₂+6H₂O   II

6NO+4NH₃→5N₂+6H₂O   III

6NO₂+8NH₃→7N₂+12H₂O   IV

NO+NO₂+2NH₃→2N₂+3H₂O   V

As shown in FIG. 4, in a typical combustion process, flue gases emergingfrom a furnace (1) pass through a pipe (20) into a hot operatingelectrofilter (2) where they are freed of dust. An ammonia/air-mixtureis then introduced into contact with the gases through injector (3), andis distributed homogeneously in the flow of the exhaust gas downstreamfrom the filter (2). The mixture is then fed through pipe (22) into acatalytic reduction reactor (4).

It is shown in FIG. 4 that the catalyst (7) in the reactor (4) may be avertical array of catalyst beds, a first series of beds (5) beingpositioned above a second series of beds (6). It is possible, ifdesired, to position a gas analyzer between individual catalyst beds, orbetween the first and second series of beds 5, 6. The catalyst may be inthe form, for example, of monolithic, ceramic honeycomb catalystsdisposed one behind the other to obtain the catalytic reduction ofnitrogen oxide in the exhaust gas. There is a broad range for thepermissible distances between the catalysts, or between the individualcatalyst beds, located in the reactor (4). The dimensions of the spacingarrangement of the catalysts or catalyst beds are determined to insurethe production of a turbulent transverse movement of gas in the conduitand avoidance of local mixing or “channeling”.

From the reduction reactor (4), the gas mixture may, if desired, betransported through pipe 24 to a sulfur oxide scrubber (8) whereinsulfur oxide is reacted with water or dilute aqueous sulfuric acid toform concentrated H₂SO₄. The completely purified exhaust gas leaving thescrubber (8) may then be transported by pipe (26) to chimney (9) fordischarge into the atmosphere. In FIG. 4, the exhaust is emitted fromits source, the furnace (1), and is transported through piping and othercomponents to its ultimate destination, the chimney (9), for dischargeinto the atmosphere. The direction of flow from the furnace (1) to thechimney (9) is considered to be downstream, and the opposite directionis considered to be upstream. The piping and other components throughwhich the exhaust gas mixture is transported, and in which the reactionof reduction occur, together provide a conduit for the flow, transport,handling and disposition of the gas mixture. A gas analyzer, or the gassensing component(s) thereof, can be positioned at any location alongthis conduit, whether in a pipe or within a component such as thecatalyst (7) located in reactor 4. Multiple catalyst beds areillustrated in the apparatus of FIG. 4, and in similar fashion, theapparatus may contain a plurality of catalysts as well.

Alternatively, as shown in FIG. 5, a dust filter (2) may be locateddownstream from a catalyst (7). In a further alternative, as shown inFIG. 6, a gas mixture to be denitrified may pass horizontally through areactor 30 containing one or more catalysts or catalyst beds. Asdescribed above, multiple catalysts and/or catalyst beds may be employedin this horizontal configuration, and one or more gas analyzers may belocated between each of the catalysts and/or catalyst beds.

In the method according to the invention, essentially all catalysts maybe used which are suitable for the selective reduction of nitrogenoxide. Examples of these are activated carbon, or catalysts that aremixtures of the oxides of iron, titanium (e.g. a manganese-based TiO₂),tungsten, vanadium and molybdenum (see, for example, DE 24 58 888, whichis incorporated in its entirety as a part hereof for all purposes) orcatalysts formed of natural or synthetic aluminum silicates, forexample, zeolites (ZSM-5), or catalysts which contain precious metals ofthe platinum group. For example, a flue gas stream containing nitrogenoxides and sulphur oxides may be passed through a catalyst bedcontaining a catalyst consisting essentially of 3 to 15% by weightvanadium pentoxide (V₂O₅) on a carrier consisting of titanium dioxide(TiO₂), silica (SiO₂), and/or alumina (Al₂O₃).

The catalyst for nitrogen oxide reduction may be of any geometricalshape, such as in the form of a honeycomb monolith or in pellet orparticulate form. However, a catalyst shape resulting in a large voidand with parallel gas channels in the catalyst bed, such as a honeycombcatalyst, is preferred since the conduit gas often contains considerableamounts of dust which otherwise might clog the catalyst bed. Thehoneycomb form offers lower back pressure and a simpler possibility forcleaning off dust. A denitrification catalyst could be made for exampleas a carrier catalyst consisting of mullite honeycomb bodies of thedimensions 150 mm×150 mm×150 mm length with a cell density of 16/cm² anda zeolite coating of the mordenite type. A moving bed is typically usedfor granular activated carbon.

The catalyst can consist completely of a catalytically active mass(solid catalyst), or the catalytically active substance can be depositedon an inert, ceramic or metallic body, which optionally can be coated inaddition with a surface area enlarging oxide layer (carrier catalyst).For example, the catalyst may be in the form of a solid-bed reactor witha flow directed preferably vertically downward. The reactor may containa honeycomb structure, which has a crystalline vanadium-titaniumcompound as the catalytically active substance. The pressure loss in thesolid-bed reactor is taken into account in establishing the size of theconduit gas blower. The vertically downward flow in the reactor isintended to combat the depositing of solid impurities within thecatalyst or keep them within acceptable ranges. The incrustation thatoccurs is removed discontinuously by blasting with compressed air orsteam.

The catalytic reaction, preferably carried out in a single reactor, maybe operated in the temperature range of about 250-550° C., preferablyabout 350-450° C., and more preferably about 380-420° C. The temperatureshould not be so high that the reducing agent is degraded (as in theconversion, for example, of ammonia into NOx and water), or so low thatthe reducing agent does not fully react with the emitted NOx, isreleased into the atmosphere and becomes a pollutant itself. The molarratio of reducing agent to nitrogen oxides is typically in the range ofabout 0.6-1.8, and preferably about 1.0-1.4. In the case of a full loadoperation in a facility containing a combustion source such as anelectrical generating plant, a flue gas temperature of 350-400° C. maybe easily reached, and these are temperatures at which denitrificationcatalysts can be utilized. In the case of a variable load operation, theflue gas temperature drops as a rule below the minimum required for theoperation of the catalyst in the partial load area, so that a bypassconnection system is typically necessary for the branching off of fluegas before the last step of heat removal in the boiler in order tomaintain the reaction temperature.

Operations that are carried out in the zone of high dust lead, moreover,to catalyst abrasion by the conduit dust, and may cause deposits andthus plugging up of the catalyst channels or pores. To prevent suchcomplications, a cleaning by blowing off with (for example) hot steam isrequired at relatively short time intervals. It is preferred, however,that the reduction step be carried out using an exhaust gas which haslittle dust content or from which the dust has been largely removedbecause the mechanical and thermal load of the catalyst is considerablyless. For the removal of the dust, the use of a high temperatureelectrofilter is particularly suitable. A filter of this type requiresslightly higher investments in comparison to a cold operatingelectrofilter, but reheating measures and problems which are connectedwith the catalyst abrasion are avoided. Both embodiments in additionhave the advantage that the removal dust is not contaminated withreducing agent.

To obtain an efficient decrease in the content of the nitrogen oxides ina flue gas, one approach as noted above, has been to add reducing agentin excess of the stoichiometric amount needed according to reactionsI-V. If the reducing agent is not completely converted in thedenitrification reaction, however, and a small quantity of it(designated as “ammonia slip” if the reducing agent is ammonia) ispresent in the exhaust gas after it is emitted into the atmosphere, theusual goal of limiting the content of reducing agent in treated flue gasto an acceptable level, such as 5-10 ppm by volume, will not be met. Thealternative of utilizing less than stoichiometric amounts of reducingagent, and compensating by the use of increased volumes of catalyst,will increase the catalyst costs. The efficiency of the denitrificationprocess will, moreover, be decreased as the absence of a stoichiometricamount of reducing agent will be the limiting factor in the reaction,and reduction of nitrogen oxides at an acceptable level will not occur.The methods and apparatus of this invention are used to furnishinformation about the compositional content of the gas mixture beingsubjected to denitrification to enable determination of the correctamount of reducing agent to be injected into the gas mixture, therebydecreasing the release of unreacted reducing agent.

For the purpose of controlling the denitrification reaction, it is alsodesirable to evaluate the success of the reaction by determininginformation about the compositional content of the gas mixture before itis emitted into the atmosphere. This type of determination may be made,for example, at one or more positions after the gas mixture has passedthe point of injection of the reducing agent, if the reaction isuncatalyzed, or after the gas mixture has passed downstream from areducing reactor if the reaction is catalyzed. Alternatively, if anoxidation catalyst is provided to oxidize unreacted reducing agent, thecompositionally-related information may be determined at one or morepositions after the gas mixture has passed downstream from the oxidationcatalyst.

When such an oxidation catalyst is employed, and the reducing agent isfor example, ammonia, ammonia is oxidized to nitrogen and wateraccording to the following reaction:

4NH₃+3O₂→6H₂O+2N₂   VI

Typical oxidation catalysts for this purpose are based on transitionmetals, for example those containing oxides of copper, chromium,manganese and/or iron. A catalyst consisting essentially of about 2 to7% by weight vanadium promoted with at least one alkali metal in avanadium to alkali metal atomic ratio in the range from about 1:2 to 1:5on a silica carrier is advantageously employed since this catalyst givesa high degree of conversion according to the reaction VI. The alkalimetal employed is preferably potassium.

One example of the manner in which the methods and apparatus of thisinvention can be used to control the reduction of a nitrogen oxide is tocontrol the injection of the reducing agent into the nitrogen oxide,such as by controlling the injection of the reducing agent into a gasmixture that contains a nitrogen oxide. In the case of nitrogen oxidethat is emitted by a source of combustion, control of the reductionreaction may be effected in terms of the compositional content of thestream of exhaust gas given off by the combustion. Information may beobtained that is related to the compositional content of the exhaust gasat points in time both before and after a reducing agent has beeninjected into the nitrogen oxide.

Information related to the compositional content of a gas mixturecontaining a nitrogen oxide may be obtained from a gas analyzer that isexposed to the gas mixture. This is most conveniently done by placingone or more gas analyzers in a conduit in which the mixture containingthe nitrogen oxide is transported from its source of emission to itseventual destination, such as discharge into the atmosphere. In the caseof exhaust gas emitted from a source of combustion, this represents achallenge because combustion exhaust gases reach high temperatures thatwill degrade the materials and instrumentation from which manyanalytical devices are made. A gas analyzer as used in this invention isone that is not degraded by, or does not malfunction as a result ofexposure to, a gas or gas mixture having a temperature of about 300° C.or more. Preferably the analyzer is not degraded or does not malfunctionat even higher temperatures such as about 400° C. or more, about 500° C.or more, about 600° C. or more, about 700° C. or more, about 800° C. ormore, about 900° C. or more, or about 1000° C. or more. The gas analyzerused in this invention, including the reactive or gas sensing componentsthereof, may thus be positioned in a gas mixture having a temperature asdescribed above, and may thus be located in the same conduit in whichthe reducing agent is injected to effect the reduction reaction.Although the analyzer as it is installed in the conduit is connected toconductors that transmit signal outputs of the analyzer elsewhere forfurther processing, the only contact between the analyzer and thenitrogen oxide to be reduced, or the gas mixture containing the nitrogenoxides occurs in the conduit in which the nitrogen oxides aretransported from their source to their eventual destination. Theanalyzer is not operated by withdrawing gas from the conduit foranalysis in a separate chamber that is outside of the conduit.

A gas analyzer that is exposed to a gas mixture containing a nitrogenoxide is used to provide information related to the compositionalcontent of the gas mixture for the purpose of controlling the reductionreaction. The information is used, in particular to control theinjection of the reducing agent into the nitrogen oxide, such as bycontrolling the injection of the reducing agent into the gas mixturecontaining the nitrogen oxide. Information as to the compositionalcontent of the gas mixture obtained before the reducing agent has beeninjected, or before the gas mixture has contacted a catalyst (if acatalyst is used), may be used to assist in the calculation of astoichiometrically correct amount of reducing agent. This“stoichiometrically correct” amount is an amount that is sufficient toreact with all nitrogen oxides present in the mixture without providingan excess of reducing agent that will be transported downstream with themixture as a pollutant itself. Information as to the compositionalcontent of the gas mixture obtained after the reducing agent has beeninjected may be used to evaluate the accuracy of the calculation bywhich the stoichiometrically correct amount of reducing agent isdetermined. If it appears that the calculation is not accurate becausethe gas mixture downstream from the injector, and downstream from thecatalyst if a catalyst is used, contains more nitrogen oxide thandesired or more reducing agent than desired, adjustments can be made tothe calculation in view of such information obtained downstream from theposition of the reduction reaction.

FIG. 7 shows a schematic layout of one possible placement of a gasanalyzer both upstream 40 and downstream 42 from the position of areduction reactor 44 in which a catalyst is employed, also upstream 46from the point of injection of the reducing agent. By conductors 48, 50and 52, information about the compositional content of the gas mixtureis fed to a reducing agent control system 54. In addition to a pump forinjecting the reducing agent, the reducing agent control system maycontain a decision-making routine and/or a map. Information from gasanalyzer 46 may be fed forward to control system 54 to assist inperforming a first calculation of the amount of reducing agent to beinjected into the gas mixture. Information from gas analyzer 40 may befed back to control system 54 to evaluate whether the reducing agent isin place in the gas mixture to the extent and with the distribution asdesired, and, in view of such finding, to also assist in performingadjustments as needed on the original calculation of the amount ofreducing agent to be injected into the gas mixture. Information from gasanalyzer 42 may be fed back to control system 54 to evaluate whethernitrogen oxide and the reducing agent are both absent from the gasmixture to the extent desired, and, in view of such finding, to alsoassist in performing adjustments as needed on the original calculationof the amount of reducing agent to be injected into the gas mixture.

The gas source 56 could be a stationary source of combustion, such as afurnace or a boiler for a steam turbine; a source of combustion that canbe stationary, mobile or self-propelled such as a gas turbine or aninternal combustion engine; or a chemical reaction that does not involvecombustion such as an industrial process. Although ammonia is shown asthe reducing agent, other reducing agents such as urea are also useful.

To control the operation of the reducing agent injector, the reducingagent control system performs certain decision-making routines aboutvarious operating characteristics of the reaction of reduction. The gasanalyzers provide information to the control system about operatingcharacteristics such as the amount and rate of injection of the reducingagent, about the presence of the reducing agent in the gas mixturebefore the reaction occurs, and about the success of the reaction interms of the extent of presence of nitrogen oxide and/or reducing agentin the gas mixture after the reaction is completed. The reducing agentcontrol system controls the injection of reducing agent by calculatingan initial amount of reducing agent needed in view of the amount ofnitrogen oxide determined to be present in the gas mixture, and byadjusting that calculation depending on the extent to which the reducingagent is successfully incorporated into the gas mixture before thereaction occurs, and depending on the extent to which nitrogen oxide hasbeen reacted out of the gas mixture without reducing agent slip.

The decision-making routine in the reducing agent control system is runby a microprocessor chip, and applies one or more algorithms and/ormathematical operations to that information to obtain a decision in theform of a value that is equivalent to a desired state or condition thatshould be possessed by a particular operating characteristic. Based onthe result of a decision-making routine, instructions are given by thereducing agent control system that cause a change in the rate or amountof injection of reducing agent thus moving the reduction reaction asclose as possible to ideal performance, which is characterized byminimal residual nitrogen oxide and minimal reducing agent slip. In apreferred embodiment of this invention, a gas mixture that contains anitrogen oxide that is reduced is, after the reduction reaction, free orsubstantially free of nitrogen oxide, and/or is free or substantiallyfree of reducing agent.

In performing a decision-making routine, the reducing agent controlsystem may, and preferably does, employ a map. A map resides in aread-only memory, and is an electronic collection of information aboutvarious operating characteristics of the reaction of reduction. In oneembodiment, a range of quantified values may be set forth within the mapwith respect to a particular operating characteristic. This could be,for example, a range of temperature between 350 and 750° C., dividedinto 25° C. increments. With respect to each individual value of theparameter or operating characteristic in the range set forth, the mapmay then associate an acceptable value for one or more other operatingcharacteristics, or a factor to be used in a decision-making routine. Amap can be established in the form of a relational database, and can beaccessed by look-up instructions in a computer program.

In the performance of a decision-making routine to control the operationof the reaction of reducing a nitrogen oxide, a value, such as the sizeof an electrical signal, that is representative of the state orcondition of operating characteristic A may be inputted to the reducingagent control system. In one example of how the signal can then beutilized by a decision-making routine, the microprocessor chipdetermines a value representative of the state or condition each ofoperating characteristics B and C, and reads the map to determine, inview of the values for B and C, a target value D for operatingcharacteristic A. The target value could be a preselected value that isrecorded in the map as such, or could be a value that is calculated bythe reducing agent control system by a mathematical operation recordedin the map, with the calculation to specify D being made only on theoccasion when the values for B and C are determined. For example, adetermination may be made of the absolute value of the differencebetween A and B, and this absolute value, when added to C, becomes thetarget value D.

The value of operating characteristic A is compared to target value D,and if A is in a desired relationship to D, the reducing agent controlsystem does not instruct that any adjustment in operations be made. If Ais not in a desired relationship to D, the decision-making processcould, in further alternative embodiments, read the map to determine adesired value or range of values for A in terms of values for operatingcharacteristics E and F; or calculate a desired value for A by readingthe map to determine coefficients to be used in performing amathematical operation on E and F. The values for E and F could bedetermined at the time of making the decision, or could be preselectedvalues stored in the map. In either case, once the desired value for Ais determined, the reducing agent control system instructs the necessaryoperating characteristics of the reaction of reduction to be adjusted inthe manner necessary to obtain the desired value for A. This may be doneby adjusting operating characteristic A itself, or adjusting otheroperating characteristics that can influence the state or condition ofA. For example, the reaction of reduction may be controlled by adjustingthe amount or frequency of injection of reducing agent, by adjusting thetiming of injection by injectors in different locations, by heating orcooling the gas mixture or a reduction catalyst, and/or by adjusting theoperation of the emissions source such as by adjusting the fuel to airratio in a combustion reaction.

In this invention, information about the compositional content of thegas emitted by a chemical reaction, such as the exhaust gas of a sourceof combustion, may be used as an input to a decision-making in thereducing agent control system. In the example described above,information about combustion exhaust gas could be used as therepresentative value that is inputted with respect to any one or more ofoperating characteristics A, B, C, E or F, or could be used as acoefficient in a operation that the decision-making routine causes to beperformed. Information about the gas composition is inputted to thedecision-making routine, in this invention, in the form of one or moresignals that is or are related to the individual concentration withinthe emitted gas stream of a particular individual component gas therein,or a particular subgroup of some but not all of the component gasestherein, or both an individual component and a subgroup. Therelationship may be a mathematical relationship, such as a monotonicrelationship, involving for example a log, inverse or scaled value. Thisis accomplished by exposing a gas analyzer, such as an array ofchemo/electro-active materials, to the emitted gas stream to generatethat may be, for example, an electrical or optical signal.

The ability to furnish information about the individual concentrationwithin an emitted gas stream of a particular component gas or subgrouptherein makes it possible to calibrate a map. When building a map beforea reaction or device to be controlled is put into service, valuesrepresentative of a variety of parameters or operating characteristicsmust be determined by systematically operating the reaction or deviceunder a large enough sample of different conditions to approximate allthe conditions expected in actual service. A gas analyzer, such as anarray of chemo/electro-active materials, can be used to analyze thecomposition of the emitted gas stream to furnish information based onthe concentration of individual components or subgroups therein to berecorded in the map in relation to values of other parameters oroperating characteristics measured under the same operating conditions.

If preferred, however, this ability to furnish information related tothe concentration of individual components or subgroups in an emittedgas stream can be used to calibrate or re-calibrate a map in real timewhile the reduction reaction is in service. For example, a relationshipcould be established in a map between a value representative of theconcentration of an individual gas component or subgroup, and valuesrepresentative of various parameters or operating characteristics, withthe value for the gas concentration to be supplied in real time. Thismight take the form of a decision-making routine involving amathematical operation in which a value representative of theconcentration of an individual gas component or subgroup is used as afactor or coefficient. The value representative of the concentration ofan individual gas component or subgroup could remain undetermined untilthe time that the mathematical operation is performed in the executionof the decision-making routine to make the decision. The valuerepresentative of the concentration of an individual gas component orsubgroup is determined and supplied to the decision-making routine onlyon the occasion of making the decision, and the decision thus need notbe made based on information that may not be currently accurate at thetime the decision is made. A map in which one or more parameters oroperating characteristics is related to information about theconcentration of an individual gas component or subgroup, with theinformation about the gas concentration being furnished in real timewhile a reaction or device is in service, clearly then has substantialvalue because it is possible to essentially recalibrate the mapcontinually in real time.

In this invention, information about an emitted gas composition may besupplied to a map from a a gas analyzer employing one or morechemo/electro-active materials that furnishes an analysis of the emittedgas stream. Responses generated by the gas analyzer are then used asinputs, optionally along with the input from other sensors such as atemperature sensor, in the operation of algorithms that control thereaction of reduction.

In the case again of an engine, there are several ways in which a gasanalyzer, such as an apparatus containing one or morechemo/electro-active materials, can be incorporated into the operationof a reducing agent control system to control the injection of reducingagent and to control, ultimately, the reaction of reduction. Thechemo/electro-active materials may be constructed as an array of sensorsthat have sensitivity to individual gaseous components or subgroups ofgases in a multi-component gas mixture, such as a stream of exhaust.Such sensors can be fabricated from semiconducting materials thatrespond uniquely to individual gases or gas subgroups that have commoncharacteristics such as similar oxidation potential, electronegativity,or ability to form free radicals. These are properties of interest whencharacterizing combustion.

Typical examples of individual gases and subgroups of gases within anexhaust stream from a combustion reaction include oxygen, carbonmonoxide, hydrogen, sulfur dioxide, ammonia, CO₂, H₂S, methanol, water,a hydrocarbon (such as C_(n)H_(2n+2), and as same may be saturated orunsaturated, or be optionally substituted with hetero atoms; and cyclicand aromatic analogs thereof), a nitrogen oxide (such as NO, NO₂, N₂O orN₂O₄) or an oxygenated carbon (CO, CO₂ or C₅O₃). The responses of anarray of chemo/electro-active materials to the multi-component mixtureof such gases formed by a stream of exhaust can thus be used todetermine what type of control over a reaction of reduction is needed toexecute a reaction in which nitrogen oxide content is decreased to thegreatest extent possible without engendering unacceptable reducing agentslip.

As an example, FIGS. 8 and 9 show several possible locations of a gasanalyzer, such as an array of sensor materials, in the exhaust system ofa vehicular internal combustion engine. The engine in FIGS. 8 and 9contains a mass airflow and outside temperature sensor 60, an idle airvalve 62, a throttle position valve 64, an exhaust gas recycle valve 66,an air temperature sensor 68, a pressure sensor 70, an air intake 72, anintake manifold 74, fuel injectors 76, spark plugs 78, a crank positionsensor 80, a cam position sensor 82, a coolant temperature sensor 84, apre-catalytic converter 86, an emissions control device (such as acatalytic converter and/or a device for the storage or abatement of NOx)90, and a temperature sensor 92. The temperature sensor shown in FIGS. 8and 9 need not be located adjacent the emissions control device 90 orthe SCR catalyst 104, or additional temperature sensors may be locatedelsewhere along the exhaust conduit. FIG. 8 shows three possiblelocations 94, 96, 98 for a gas analyzer, which may be upstream ordownstream from the emissions control device. The arrows indicate thelocations where it would be possible, if desired, to provide for theflow of information to/from an engine control unit to/from one or moresensors or actuators.

A gas analyzer at position 94 is located close to engine and respondsdirectly to the exhaust from individual cylinders. Because of itsproximity and fast response, the array in this location can be used toobtain information from, or to control the operation of, each individualcylinder. An array in this location is exposed to very high exhausttemperatures for which semiconducting sensor materials are verysuitable. A gas sensor in position 96 in FIG. 8 operates cooler and isexposed to gases that have already been modified in composition by theprecatalyst. However, the gas stream at this point still contains muchchemical information that can be used for control the reduction ofnitrogen oxides. This is also a suitable location to employ feed-forwardcontrol by using an array of sensor materials to control operation ofthe catalytic converter, which catalyzes the completion of the oxidationof unburned fuel. Position 98 is a location that can be used to monitorengine emissions and the current state of the catalytic converter. Basedon information from gas analyzer at this location, the catalyticconverter can be regenerated or otherwise controlled through feedbackprocess control.

FIG. 9 shows an SCR catalyst 104 and the deployment of gas sensors in acontrol system in which a reducing agent is injected into the exhaustconduit at position 110. Reducing agent is supplied from a reservoir 102and is passed through reducing agent control system 100 for injectioninto the exhaust conduit. Reducing agent control system 100 includes thenecessary pump to inject the reducing into the exhaust conduit, and isconnected to the microprocessor chip for the passage of signals to andfrom the microprocessor chip to control the injection of reducing agent.A gas analyzer, such as a gas sensor, can in this arrangement be usedeither for feed-forward (position 106) or feedback (position 108)control. The gas sensor is responsive to a variety of gases that may bepresent in a combustion exhaust stream such as ammonia, nitrogen oxide,carbon monoxide, oxygen, hydrocarbons and water. The reducing agentcontrol system, and the injection of reducing agent, may be controlledby information obtained from a gas analyzer that is positioned bothupstream and/or downstream from a reduction catalyst and, optionally,upstream and/or downstream from the reducing agent injector. Informationabout the compositional content of the gas mixture containing a nitrogenoxide is provided to a decision-making routine and/or map in themicroprocessor chip for processing into signals routed to the reducingagent pump, to the engine itself or to heating or cooling devices forthe purpose of controlling the reaction of reduction.

An internal combustion engine, in which nitrogen oxide reduction iscontrolled by the methods and apparatus of this invention, can be usedfor many different purposes including, for example, in any type ofvehicle for transportation or recreation such as a car, truck, bus,locomotive, aircraft, spacecraft, boat, jet ski, all-terrain vehicle orsnowmobile; or in equipment for construction, maintenance or industrialoperations such as pumps, lifts, hoists, cranes, generators, orequipment for demolition, earth moving, digging, drilling, mining orgroundskeeping.

Although this invention has been described in detail with respect to thecontrol of the reduction of nitrogen oxides generated by combustion,i.e. the oxidation of a fossil fuel, it is equally applicable to thereduction of nitrogen oxides that may be found in a gas mixturegenerated by any other type of chemical reaction. It is also equallyapplicable to the reduction of nitrogen oxides that are not in a mixturewith other gases, where, for example, a gas analyzer is used todetermine information related to the relative concentration within thegroup of nitrogen oxide of each individual nitrogen oxide. It is alsoequally applicable to reducing agents in addition to ammonia and urea.

It will thus be seen that, in various embodiments of this invention, asthere may a plurality of reducing agent injectors, one or more gasanalyzers may be located in the conduit upstream or downstream from eachreducing agent injector. If a dust filter is used, it may be locatedupstream from a reducing agent injector and/or one or more gasanalyzers.

If a catalyst is present, the catalyst may be located upstream ordownstream from one or more gas analyzers. A first catalyst may belocated upstream from one or more gas analyzers, and a second catalystmay be located downstream from one or more gas analyzers, particularlywhere the catalyst is a plurality of vertically arranged catalyst beds.A first gas analyzer may be located upstream from a catalyst, and asecond gas analyzer may be located downstream from the catalyst. One ormore gas analyzers may be located between first and second catalysts.One or more gas analyzers may be located at the point of destination ofa flowing stream of a gas mixture, such as at a point of discharge tothe atmosphere.

If a gas analyzer outputs a signal to a decision-making routine, a gasanalyzer that is upstream from all catalyst, a gas analyzer that isdownstream from a first catalyst and upstream from a second catalyst,and/or a gas analyzer that is downstream from all catalyst may eachoutput a signal to a decision-making routine. A gas analyzer may outputat least one signal that is related to the individual concentrationwithin the gas mixture of an individual nitrogen oxide componenttherein, and/or may output at least one signal that is related to thecollective concentration within the gas mixture of all nitrogen oxidecomponents therein. The gas analyzer may in turn output a signal to amap. The gas analyzer may also output a signal to a decision-makingroutine that controls the injection of reducing agent, such as bycalculating an amount of reducing agent to be injected.

Information as to the compositional content of a gas mixture may bedetermined before the injection of reducing agent, and/or before the gasmixture contacts any catalyst. Information as to the compositionalcontent of a gas mixture may also be determined after the gas mixturecontacts a first catalyst but before the gas mixture contacts a secondcatalyst, or after the gas mixture has contacted all catalyst. Forexample, a gas analyzer that is upstream from all catalyst, and a gasanalyzer that is downstream from all catalyst may each output separatesignals to a decision-making routine.

The injection of the reducing agent may be controlled in relation tosuch information as to the compositional content of the gas mixture,such as by determining the amount of reducing agent to be injected intothe gas mixture. The information as to the compositional content of thegas mixture may be an output of one or more gas analyzers, and may berelated to the individual concentration within the gas mixture of anindividual gas component therein (such as a nitrogen oxide), and/orrelated to the collective concentration within the gas mixture of asubgroup of the component gases therein (such as all nitrogen oxides).

In the present invention, an array of chemo/electro-active materials isused for directly sensing one or more analyte gases in a multi-componentgas system under variable temperature conditions. By “directly sensing”is meant that an array of gas-sensing materials will be exposed to amixture of gases that constitutes a multi-component gas system, such asin a stream of flowing gases. The array may be situated within the gasmixture, and more particularly within the source of the gas mixture, ifdesired. Alternatively, although not preferred, the array may reside ina chamber to which the gas mixture is directed from its source atanother location. When gas is directed to a chamber in which an array islocated, the gas mixture may be inserted in and removed from the chamberby piping, conduits or any other suitable gas transmission equipment.

A response may be obtained upon exposure of the gas-sensing materials tothe multi-component gas mixture, and the response will be a function ofthe concentrations of one or more of the analyte gases themselves in thegas mixture. The sensor materials will be exposed simultaneously (orsubstantially simultaneously) to each of the analyte gases, and ananalyte gas does not have to be physically separated from themulti-component gas mixture to be able to conduct an analysis of themixture and/or one or more analyte components thereof. This inventioncan be used, for example, to obtain responses to, and thus to detectand/or measure the concentrations, of combustion gases, such as oxygen,carbon monoxide, nitrogen oxides, hydrocarbons such as butane, CO₂, H₂S,sulfur dioxide, halogens, hydrogen, water vapor, an organo-phosphorusgas, and ammonia, at variable temperatures in gas mixtures such asautomobile emissions.

This invention utilizes an array of sensing materials to analyze a gasmixture and/or the components thereof to, for example, obtain a responseto, detect the presence of and/or calculate the concentration of one ormore individual analyte gas components in the system. By “array” ismeant at least two different materials that are spatially separated, asshown for example in FIG. 1. The array may contain, for example, 3, 4,5, 6, 8, 10 or 12 gas-sensing materials, or other greater or lessernumbers as desired. It is preferred that there be provided at least onesensor material for each of the individual gases or subgroups of gasesin the mixture to be analyzed. It may be desirable, however, to providemore than one sensor material that is responsive to an individual gascomponent and/or a particular subgroup of gases in the mixture. Forexample, a group of at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12sensors could be used to detect the presence of, and/or calculate theconcentration of, one or more individual component gases and/or one ormore subgroups of gases in the mixture. Groups of sensors, which may ormay not have members in common, could be used to obtain a response to ananalyte that is an individual gas component or a subgroup of gases inthe mixture. A subgroup of gases that is, as the subgroup, an analytemay or may not contain as a member an individual gas that is itself alsoan analyte.

This invention is useful for detecting those gases that are expected tobe present in a gas stream. For example, in a combustion process, gasesthat are expected to be present include oxygen, nitrogen oxides (such asNO, NO₂, N₂O or N₂O₄), carbon monoxide, hydrocarbons (such asC_(n)H_(2n+2), and as same may be saturated or unsaturated, or beoptionally substituted with hetero atoms; and cyclic and aromaticanalogs thereof), ammonia or hydrogen sulfide, sulfur dioxide, CO₂, ormethanol. Other gases of interest may include alcohol vapors, solventvapors, hydrogen, water vapor, and those deriving from saturated andunsaturated hydrocarbons, ethers, ketones, aldehydes, carbonyls,biomolecules and microorganisms. The component of a multi-component gasmixture that is an analyte of interest may be an individual gas such ascarbon monoxide; may be a subgroup of some but not all of the gasescontained in the mixture, such as the nitrogen oxides (NO_(x)) orhydrocarbons; or may be a combination of one or more individual gasesand one or more subgroups. When a subgroup of gases is an analyte, achemo/electro-active material will respond to the collectiveconcentration within a multi-component gas mixture of the members of thesubgroup together.

The analyte gas(es) contained in the mixture to which thechemo/electro-active material will be exposed can be a single gas, asubgroup of gases together, or one or more gases or subgroups mixed withan inert gas such as nitrogen. Particular gases of interest are donorand acceptor gases. These are gases that either donate electrons to thesemiconducting material, such as carbon monoxide, H₂S and hydrocarbons,or accept electrons from the semiconducting material, such as O₂,nitrogen oxides (commonly depicted as NO_(x)), and halogens. Whenexposed to a donor gas, an n-type semiconducting material will have adecrease in electrical resistance, increasing the current, and it,therefore, will show an increase in temperature due to I²R heating. Whenexposed to an acceptor gas, an n-type semiconducting material will havean increase in electrical resistance, decreasing the current, andtherefore will show a decrease in temperature due to I²R heating. Theopposite occurs in each instance with p-type semiconducting materials.

Obtaining information related to the compositional content of a gasmixture using these sensor materials, such as measurement of gasconcentrations, can be based on a change in an electrical property, suchas AC impedance, of at least one, but preferably each and all, of thematerials upon exposure of the materials to a mixture containing one ormore analyte gases. Analysis of a gas mixture can also be performed interms of extent of change in other electrical properties of the sensormaterials, such as capacitance, voltage, current or AC or DC resistance.Change in DC resistance may be determined, for example, by measuringchange in temperature at constant voltage. The change in one of theseillustrative properties of a sensor material is a function of thepartial pressure of an analyte gas within the gas mixture, which in turndetermines the concentration at which the molecules of the analyte gasesbecome adsorbed on the surface of a sensor material, thus affecting theelectrical response characteristics of that material. By using an arrayof chemo/electro-active materials, a pattern of the respective responsesexhibited by the materials upon exposure to a mixture containing one ormore analyte gases can be used to simultaneously and directly detect thepresence of, and/or measure the concentration of, at least one gas in amulti-component gas system. The invention, in turn, can be used todetermine the composition of the gas system. The concept is illustratedschematically in FIG. 1 and is exemplified below.

To illustrate, consider the theoretical example below of the exposure ofa sensor material to a mixture containing an analyte gas. Where aresponse is obtained, the event is depicted as positive (+), and whereno response is obtained, the event is depicted as negative (−). Material1 responds to Gas 1 and Gas 2, but shows no response to Gas 3. Material2 responds to Gas 1 and Gas 3, but shows no response to Gas 2, andMaterial 3 responds to Gas 2 and Gas 3, but shows no response to Gas 1.

Material 1 Material 2 Material 3 Gas 1 + + − Gas 2 + − + Gas 3 − + +

Therefore, if an array consisting of Materials 1, 2 and 3 gives thefollowing response to an unknown gas,

Material 1 Material 2 Material 3 Unknown Gas + − +then the unknown gas would be identified as Gas 2. The response of eachsensor material would be a function of the partial pressure within themixture of, and thus the concentration of, an analyte gas or thecollective concentration of a subgroup of analyte gases; and theresponse could be quantified or recorded as a processible value, such asa numerical value. In such case, the values of one or more responses canbe used to generate quantitative information about the presence withinthe mixture of one or more analyte gases. In a multicomponent gassystem, chemometrics, neural networks or other pattern recognitiontechniques could be used to calculate the concentration of one or moreanalyte gases in the mixture of the system.

The sensing materials used are chemo/electro-active materials. A“chemo/electro-active material” is a material that has an electricalresponse to at least one individual gas in a mixture. Some metal oxidesemiconducting materials, mixtures thereof, or mixtures of metal oxidesemiconductors with other inorganic compounds are chemo/electro-active,and are particularly useful in this invention. Each of the variouschemo/electro-active materials used herein preferably exhibits anelectrically detectable response of a different kind and/or extent, uponexposure to the mixture and/or an analyte gas, than each of the otherchemo/electro-active materials. As a result, an array of appropriatelychosen chemo/electro-active materials can be used to analyze amulti-component gas mixture, such as by interacting with an analyte gas,sensing an analyte gas, or determining the presence and/or concentrationof one or more analyte gases or subgroups in a mixture, despite thepresence therein of interfering gases that are not of interest.Preferably the mole percentages of the major components of eachgas-sensing material differs from that of each of the others.

The chemo/electro-active material can be of any type, but especiallyuseful are semiconducting metal oxides such as SnO₂, TiO₂, WO₃ and ZnO.These particular materials are advantageous due to their chemical andthermal stability. The chemo/electro-active material can be a mixture oftwo or more semiconducting materials, or a mixture of a semiconductingmaterial with an inorganic material, or combinations thereof. Thesemiconducting materials of interest can be deposited on a suitablesolid substrate that is an insulator such as, but not limited to,alumina or silica and is stable under the conditions of themulti-component gas mixture. The array then takes the form of the sensormaterials as deposited on the substrate. Other suitable sensor materialsinclude single crystal or polycrystalline semiconductors of the bulk orthin film type, amorphous semiconducting materials, and semiconductormaterials that are not composed of metal oxides.

In various embodiments, the substrate may be a high-temperaturemultilayer ceramic, which is prepared from Al₂O₃, AlN, and, to a smallerextent, BeO and SiC. The alumina content is dominant, however, withabout 92-96 weight % of the composition being Al₂O₃. The structureconsists of many layers of ceramic, with metallization between thelayers, and via holes through the layers for electrical contact. A wellknown application of large modules with many layers of ceramic is IBM'spioneering product “Thermal Conduction Module” (TCM) for mainframecomputers in 1983. The module had 33 layers, and 133 silicon chips weremounted by flip chip soldering.

This type of non-sintered, pliable ceramic consists of alumina powder,organic binders and solvents. The material is spread from a containerdown on a transport carrier underneath. The ceramic “tape” (“greensheet”) is given the appropriate thickness on the transport carrier bypassing underneath a “doctor blade” in a precisely controlled distance.The tape is cut to correct size, and holes and component cavities arepunched out with a numerically controlled punching tool, or with apermanent, product specific punching tool for high production volume ofa given product. Metallization of the via holes and fabrication ofconductors is done by screen printing of tungsten (or molybdenum). Theseare the only metals that can withstand the high process temperatureduring the subsequent sintering process. All layers are laminatedtogether under hydrostatic (or uni-axial) pressure at elevatedtemperature (500-600° C.) to evaporate the binder and solvent. Then thewhole structure is sintered at 1370-1650° C., 30-50 hours, in a hydrogenatmosphere.

For small circuits, many modules are made on one substrate, and theindividual circuits can be parted by breaking the substrate at the endof the process. Then the external contacts are brazed to the substrate,and finally gold may be plated on the surface with nickel as a diffusionbarrier on top of the tungsten. The plating is preferably doneelectrolytically to achieve sufficient thickness and good conductivityif an electrical contact to all parts of the conductor pattern can bemade. Otherwise, chemical plating is used.

During the process, the ceramic shrinks approximately 18% linearly. Thisis taken into consideration during the design of the circuit, bothsideways and in thickness, which affects the characteristic impedance.As the shrinkage is material and process dependent, the finishedcircuits typically have linear dimensional tolerances 0.5-1%. Theseceramic substrates have low TCE, a good thermal match to Si and GaAs aswell as to leadless SMD components, good control over characteristicimpedance, and good high frequency properties. Many layers are possiblewith high production yield because each layer can be inspected beforethe lamination, and faulty layers can be discarded. Among thedisadvantages are low electrical conductivity in the inner layers (sheetresistivity˜15 mohm/sq), and high dielectric constant, which givesdelay, inferior pulse rise time and increased power loss and cross talkat very high frequencies.

The chemo/electro-active materials that contain more than one metal donot have to be a compound or solid solution, but can be a multi-phasephysical mixture of discrete metals and/or metal oxides. As there willbe varying degrees of solid state diffusion by the precursor materialsfrom which the chemo/electro-active materials are formed, the finalmaterials may exhibit composition gradients, and they can be crystallineor amorphous. Suitable metal oxides are those that

-   -   i) when at a temperature of about 400° C. or above, have a        resistivity of about 1 to about 10⁶ ohm-cm, preferably about 1        to about 10⁵ ohm-cm, and more preferably about 10 to about 10⁴        ohm-cm;    -   ii) show a chemo/electro response to at least one gas of        interest; and    -   iii) are stable and have mechanical integrity, that is are able        to adhere to the substrate and not degrade at the operating        temperature.        The metal oxides may also contain minor or trace amounts of        hydration and elements present in the precursor materials.

The sensor materials may optionally contain one or more additives topromote adhesion to a substrate, or that alter the conductance,resistance or selectivity of the sensor material. Examples of additivesto alter the conductance, resistance or selectivity of the sensormaterial include Ag, Au or Pt, as well as frits. Examples of additivesto promote adhesion include frits, which are finely ground inorganicminerals that are transformed into glass or enamel on heating, or arapidly quenched glass that retains its amorphous quality in the solidstate. Frit percursor compounds are melted at high temperature andquenched, usually by rapidly pouring the melt into a fluid such aswater, or by pouring through spinning metal rollers. The precursorcompounds usually are a mechanical mixture of solid compounds such asoxides, nitrates or carbonates, or can be co-precipitated or gelled froma solution. Suitable precursor materials for frits include alkali andalkaline earth alumino-silicates and alumino-boro-silicates, copper,lead, phosphorus, titanium, zinc and zirconium. Frits as additives maybe used in amounts of up to 30 volume percent, and preferably up to 10volume percent, of the total volume of the chemo/electro-active materialfrom which the sensor is made.

If desired, the sensor materials may also contain additives that, forexample, catalyze the oxidation of a gas of interest or promote theselectivity for a particular analyte gas; or contain one or more dopantsthat convert an n semiconductor to a p semiconductor, or vice versa.These additives may be used in amounts of up to 30 weight percent, andpreferably up to 10 weight percent, of the chemo/electro-active materialfrom which the sensor is made.

Any frits or other additives used need not be uniformly or homogeneouslydistributed throughout the sensor material as fabricated, but may belocalized on or near a particular surface thereof as desired. Eachchemo/electro-active material may, if desired, be covered with a porousdielectric overlayer.

The chemo/electro-active materials used as sensor materials in thisinvention may, for example, be metal oxides of the formula M¹O_(x), M¹_(a)M² _(b)O_(x), or M¹ _(a)M² _(b)M³ _(c)O_(x); or mixtures thereof,wherein

-   -   M¹, M² and M³ are metals that form stable oxides when fired in        the presence of oxygen above 500° C.;    -   M¹ is selected from Periodic Groups 2-15 and the lanthanide        group;    -   M² and M³ are each independently selected from Periodic Groups        1-15 and the lanthanide group;    -   M¹ and M² are not the same in M¹ _(a)M² _(b)O_(x), and M¹, M²        and M³ are not the same in M¹ _(a)M² _(b)M³ _(c)O_(x);    -   a, b, and c are each independently in the range of about 0.0005        to about 1; and    -   x is a number sufficient so that the oxygen present balances the        charges of the other elements present in the        chemo/electro-active material.

In certain preferred embodiments, the metal oxide materials may includethose in which

-   -   M¹ is selected from the group consisting of Ce, Co, Cu, Fe, Ga,        Nb, Ni, Pr, Ru, Sn, Ti, Tm, W, Yb, Zn, and Zr; and/or    -   M² and M³ are each independently selected from the group        consisting of Al, Ba, Bi, Ca, Cd, Ce, Co, Cr, Cu, Fe, Ga, Ge,        In, K, La, Mg, Mn, Mo, Na, Nb, Ni, Pb, Pr, Rb, Ru, Sb, Sc, Si,        Sn, Sr, Ta, Ti, Tm, V, W, Y, Yb, Zn, and Zr;        but in which M¹ and M² are not the same in M¹ _(a)M² _(b)O_(x),        and M¹, M² and M³ are not the same in M¹ _(a)M² _(b)M³        _(c)O_(x).

In certain other preferred embodiments, the metal oxide materials mayinclude those in which

M¹O_(x) is CeO_(x), CoO_(x), CuO_(x), FeO_(x), GaO_(x), NbO_(x),NiO_(x), PrO_(x), RuO_(x), SnO_(x), TaO_(x), TiO_(x), TmO_(x), WO_(x),YbO_(x), ZnO_(x), ZrO_(x), SnO_(x) with Ag additive, ZnO_(x) with Agadditive, TiO_(x) with Pt additive, ZnO_(x) with frit additive, NiO_(x)with frit additive, SnO_(x) with frit additive, or WO_(x) with fritadditive; and/or

M¹ _(a)M² _(b)O_(x) is Al_(a)Cr_(b)O_(x), Al_(a)Fe_(b)O_(x),Al_(a)Mg_(b)O_(x), Al_(a)Ni_(b)O_(x), Al_(a)Ti_(b)O_(x),Al_(a)V_(b)O_(x), Ba_(a)Cu_(b)O_(x), Ba_(a)Sn_(b)O_(x),Ba_(a)Zn_(b)O_(x), Bi_(a)Ru_(b)O_(x), Bi_(a)Sn_(b)O_(x),Bi_(a)Zn_(b)O_(x), Ca_(a)Sn_(b)O_(x), Ca_(a)Zn_(b)O_(x),Cd_(a)Sn_(b)O_(x), Cd_(a)Zn_(b)O_(x), Ce_(a)Fe_(b)O_(x),Ce_(a)Nb_(b)O_(x), Ce_(a)Ti_(b)O_(x), Ce_(a)V_(b)O_(x),Co_(a)Cu_(b)O_(x), Co_(a)Ge_(b)O_(x), Co_(a)La_(b)O_(x),Co_(a)Mg_(b)O_(x), Co_(a)Nb_(b)O_(x), Co_(a)Pb_(b)O_(x),Co_(a)Sn_(b)O_(x), Co_(a)V_(b)O_(x), Co_(a)W_(b)O_(x),Co_(a)Zn_(b)O_(x), Cr_(a)Cu_(b)O_(x), Cr_(a)La_(b)O_(x),Cr_(a)Mn_(b)O_(x), Cr_(a)Ni_(b)O_(x), Cr_(a)Si_(b)O_(x),Cr_(a)Ti_(b)O_(x), Cr_(a)Y_(b)O_(x), Cr_(a)Zn_(b)O_(x),Cu_(a)Fe_(b)O_(x), Cu_(a)Ga_(b)O_(x), Cu_(a)La_(b)O_(x),Cu_(a)Na_(b)O_(x), Cu_(a)Ni_(b)O_(x), Cu_(a)Pb_(b)O_(x),Cu_(a)Sn_(b)O_(x), Cu_(a)Sr_(b)O_(x), Cu_(a)Ti_(b)O_(x),Cu_(a)Zn_(b)O_(x), Cu_(a)Zr_(b)O_(x), Fe_(a)Ga_(b)O_(x),Fe_(a)La_(b)O_(x), Fe_(a)Mo_(b)O_(x), Fe_(a)N_(b)O_(x),Fe_(a)Ni_(b)O_(x), Fe_(a)Sn_(b)O_(x), Fe_(a)Ti_(b)O_(x),Fe_(a)W_(b)O_(x), Fe_(a)Zn_(b)O_(x), Fe_(a)Zr_(b)O_(x),Ga_(a)La_(b)O_(x), Ga_(a)Sn_(b)O_(x), Ge_(a)Nb_(b)O_(x),Ge_(a)Ti_(b)O_(x), In_(a)Sn_(b)O_(x), K_(a)Nb_(b)O_(x),Mn_(a)Nb_(b)O_(x), Mn_(a)Sn_(b)O_(x), Mn_(a)Ti_(b)O_(x),Mn_(a)Y_(b)O_(x), Mn_(a)Zn_(b)O_(x), Mo_(a)Pb_(b)O_(x),Mo_(a)Rb_(b)O_(x), Mo_(a)Sn_(b)O_(x), Mo_(a)Ti_(b)O_(x),Mo_(a)Zn_(b)O_(x), Nb_(a)Ni_(b)O_(x), Nb_(a)Ni_(b)O_(x),Nb_(a)Sr_(b)O_(x), Nb_(a)Ti_(b)O_(x), Nb_(a)W_(b)O_(x),Nb_(a)Zr_(b)O_(x), Ni_(a)Si_(b)O_(x), Ni_(a)Sn_(b)O_(x),Ni_(a)Y_(b)O_(x), Ni_(a)Zn_(b)O_(x), Ni_(a)Zr_(b)O_(x),Pb_(a)Sn_(b)O_(x), Pb_(a)Zn_(b)O_(x), Rb_(a)W_(b)O_(x),Ru_(a)Sn_(b)O_(x), Ru_(a)W_(b)O_(x), Ru_(a)Zn_(b)O_(x),Sb_(a)Sn_(b)O_(x), Sb_(a)Zn_(b)O_(x), Sc_(a)Zr_(b)O_(x),Si_(a)Sn_(b)O_(x), Si_(a)Ti_(b)O_(x), Si_(a)W_(b)O_(x),Si_(a)Zn_(b)O_(x), Sn_(a)Ta_(b)O_(x), Sn_(a)Ti_(b)O_(x),Sn_(a)W_(b)O_(x), Sn_(a)Zn_(b)O_(x), Sn_(a)Zr_(b)O_(x),Sr_(a)Ti_(b)O_(x), Ta_(a)Ti_(b)O_(x), Ta_(a)Zn_(b)O_(x),Ta_(a)Zr_(b)O_(x), Ti_(a)V_(b)O_(x), Ti_(a)W_(b)O_(x),Ti_(a)Zn_(b)O_(x), Ti_(a)Zr_(b)O_(x), V_(a)Zn_(b)O_(x),V_(a)Zr_(b)O_(x), W_(a)Zn_(b)O_(x), W_(a)Zr_(b)O_(x), Y_(a)Zr_(b)O_(x),Zn_(a)Zr_(b)O_(x), Al_(a)Ni_(b)O_(x) with frit additive,Cr_(a)Ti_(b)O_(x) with frit additive, Fe_(a)La_(b)O_(x) with fritadditive, Fe_(a)Ni_(b)O_(x) with frit additive, Fe_(a)Ti_(b)O_(x) withfrit additive, Nb_(a)Ti_(b)O_(x) with frit additive, Nb_(a)W_(b)O_(x)with frit additive, Ni_(a)Zn_(b)O_(x) with frit additive,Ni_(a)Zr_(b)O_(x) with frit additive, Sb_(a)Sn_(b)O_(x) with fritadditive, Ta_(a)Ti_(b)O_(x) with frit additive, or Ti_(a)Zn_(b)O_(x)with frit additive; and/or

M¹ _(a)M² _(b)M³ _(c)O_(x) is Al_(a)Mg_(b)Zn_(c)O_(x),Al_(a)Si_(b)V_(c)O_(x), Ba_(a)Cu_(b)Ti_(c)O_(x),Ca_(a)Ce_(b)Zr_(c)O_(x), Co_(a)Ni_(b)Ti_(c)O_(x),Co_(a)Ni_(b)Zr_(c)O_(x), Co_(a)Pb_(b)Sn_(c)O_(x),Co_(a)Pb_(b)Zn_(c)O_(x), Cr_(a)Sr_(b)Ti_(c)O_(x),Cu_(a)Fe_(b)Mn_(c)O_(x), Cu_(a)La_(b)Sr_(c)O_(x),Fe_(a)Nb_(b)Ti_(c)O_(x), Fe_(a)Pb_(b)Zn_(c)O_(x),Fe_(a)Sr_(b)Ti_(c)O_(x), Fe_(a)Ta_(b)Ti_(c)O_(x),Fe_(a)W_(b)Zr_(c)O_(x), Ga_(a)Ti_(b)Zn_(c)O_(x),La_(a)Mn_(b)Na_(c)O_(x), La_(a)Mn_(b)Sr_(c)O_(x),Mn_(a)Sr_(b)Ti_(c)O_(x), Mo_(a)Pb_(b)Zn_(c)O_(x),Nb_(a)Sr_(b)Ti_(c)O_(x), Nb_(a)Sr_(b)W_(c)O_(x),Nb_(a)Ti_(b)Zn_(c)O_(x), Ni_(a)Sr_(b)Ti_(c)O_(x),Sn_(a)W_(b)Zn_(c)O_(x), Sr_(a)Ti_(b)V_(c)O_(x), Sr_(a)Ti_(b)Zn_(c)O_(x),or Ti_(a)W_(b)Zr_(c)O_(x).

In certain other preferred embodiments, the metal oxide materials mayinclude those that are in an array of first and secondchemo/electro-active materials, wherein the chemo/electro-activematerials are selected from the pairings in the group consisting of

-   -   (i) the first material is M¹O_(x), and the second material is M¹        _(a)M² _(b)O_(x);    -   (ii) the first material is M¹O_(x), and the second material is        M¹ _(a)M² _(b)M³ _(c)O_(x);    -   (iii) the first material is M¹ _(a)M² _(b)O_(x), and the second        material is M¹ _(a)M² _(b)M³ _(c)O_(x);    -   (iv) the first material is a first M¹O_(x); and the second        material is a second M¹O_(x);    -   (v) the first material is a first M¹ _(a)M² _(b)O_(x), and the        second material is a second M¹ _(a)M² _(b)O_(x); and    -   (vi) the first material is a first M¹ _(a)M² _(b)M³ _(c)O_(x),        and the second material is a second M¹ _(a)M² _(b)M³ _(c)O_(x);        wherein

M¹ is selected from the group consisting of Ce, Co, Cu, Fe, Ga, Nb, Ni,Pr, Ru, Sn, Ti, Tm, W, Yb, Zn, and Zr;

M² and M³ are each independently selected from the group consisting ofAl, Ba, Bi, Ca, Cd, Ce, Co, Cr, Cu, Fe, Ga, Ge, In, K, La, Mg, Mn, Mo,Na, Nb, Ni, Pb, Pr, Rb, Ru, Sb, Sc, Si, Sn, Sr, Ta, Ti, Tm, V, W, Y, Yb,Zn, and Zr;

but M¹ and M² are not the same in M¹ _(a)M² _(b)O_(x), and M¹, M² and M³are not the same in M¹ _(a)M² _(b)M³ _(c)O_(x);

a, b and c are each independently about 0.0005 to about 1; and

x is a number sufficient so that the oxygen present balances the chargesof the other elements present in the chemo/electro-active material.

In certain other preferred embodiments, an array of two or morechemo/electro-active materials may be selected from the group consistingof (i) the chemo/electro-active materials that include M¹O_(x), (ii) thechemo/electro-active materials that include M¹ _(a)M² _(b)O_(x), and(iii) the chemo/electro-active materials that include M¹ _(a)M² _(b)M³_(c)O_(x);

-   -   wherein M¹ is selected from the group consisting of Al, Ce, Cr,        Cu, Fe, Ga, Mn, Nb, Ni, Pr, Sb, Sn, Ta, Ti, W and Zn;    -   wherein M² and M³ are each independently selected from the group        consisting of Ga, La, Mn, Ni, Sn, Sr, Ti, W, Y, Zn;    -   wherein M¹ and M² are each different in M¹ _(a)M² _(b)O_(x), and        M¹, M² and M³ are each different in M¹ _(a)M² _(b)M³ _(c)O_(x);    -   wherein a, b and c are each independently about 0.0005 to about        1; and    -   wherein x is a number sufficient so that the oxygen present        balances the charges of the other elements in the        chemo/electro-active material.

M¹ may for example be selected from the group consisting of Al, Cr, Fe,Ga, Mn, Nb, Ni, Sb, Sn, Ta, Ti and Zn, or from the group consisting ofGa, Nb, Ni, Sb, Sn, Ta, Ti and Zn. M², M³, or M² and M³ may be selectedfrom the group consisting of La, Ni, Sn, Ti and Zn, or the groupconsisting of Sn, Ti and Zn.

The array may contain other numbers of chemo/electro-active materialssuch as four or eight, and the array may contain at least onechemo/electro-active material that comprises M¹O_(x), and at least threechemo/electro-active materials that each comprise M¹ _(a)M² _(b)O_(x).Alternatively, the array may contain (i) at least onechemo/electro-active material that comprises M¹O_(x), and at least fourchemo/electro-active materials that each comprise M¹ _(a)M² _(b)O_(x);or (ii) at least two chemo/electro-active materials that each compriseM¹O_(x), and at least four chemo/electro-active materials that eachcomprise M¹ _(a)M² _(b)O_(x); or (iii) at least threechemo/electro-active materials that each comprise M¹ _(a)M² _(b)O_(x),and at least one chemo/electro-active material that comprises M¹ _(a)M²_(b)M³ _(c)O_(x).

Chemo/electro-active materials useful in the apparatus of this inventionmay be selected from one or more members of the group consisting of

-   -   a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises CeO₂,    -   a chemo/electro-active material that comprises        Cr_(a)Mn_(b)O_(x),    -   a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)    -   a chemo/electro-active material that comprises        Cu_(a)Ga_(b)O_(x),    -   a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)    -   a chemo/electro-active material that comprises CuO,    -   a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ga_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Sr_(b)O_(x),    -   a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Nb_(a)W_(b)O_(x)    -   a chemo/electro-active material that comprises NiO,    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)    -   a chemo/electro-active material that comprises Pr₆O₁₁,    -   a chemo/electro-active material that comprises        Sb_(a)Sn_(b)O_(x).    -   a chemo/electro-active material that comprises SnO₂,    -   a chemo/electro-active material that comprises        Ta_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x).    -   a chemo/electro-active material that comprises WO₃, and    -   a chemo/electro-active material that comprises ZnO.        wherein a, b and c are each independently about 0.0005 to about        1; and wherein x is a number sufficient so that the oxygen        present balances the charges of the other elements in the        chemo/electro-active material.

Chemo/electro-active materials useful in this invention may also beselected from subgroups of the foregoing formed by omitting any one ormore members from the whole group as set forth in the list above. As aresult, the chemo/electro-active materials may in such instance not onlybe any one or more member(s) selected from any subgroup of any size thatmay be formed from the whole group as set forth in the list above, butthe subgroup may also exclude the members that have been omitted fromthe whole group to form the subgroup. The subgroup formed by omittingvarious members from the whole group in the list above may, moreover,contain any number of the members of the whole group such that thosemembers of the whole group that are excluded to form the subgroup areabsent from the subgroup. Representative subgroups are set forth below.

Chemo/electro-active materials that comprise M¹O_(x) may, for example,be selected from the group consisting of

-   -   a chemo/electro-active material that comprises CeO₂,    -   a chemo/electro-active material that comprises CuO,    -   a chemo/electro-active material that comprises NiO,    -   a chemo/electro-active material that comprises Pr₆O₁₁,    -   a chemo/electro-active material that comprises SnO₂,    -   a chemo/electro-active material that comprises WO₃, and    -   a chemo/electro-active material that comprises ZnO.

Of the above, one or more members of the group consisting of

-   -   a chemo/electro-active material that comprises CeO₂,    -   a chemo/electro-active material that comprises SnO₂, and    -   a chemo/electro-active material that comprises ZnO        may contain a frit additive.

A chemo/electro-active material that comprises M1aM2bO_(x), or achemo/electro-active material that comprises M1aM2bM3cOx, may beselected from the group consisting of

-   -   a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises        Cr_(a)Mn_(b)O_(x),    -   a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)    -   a chemo/electro-active material that comprises        Cu_(a)Ga_(b)O_(x),    -   a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ga_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Sr_(b)O_(x),    -   a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Nb_(a)W_(b)O_(x)    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)    -   a chemo/electro-active material that comprises        Sb_(a)Sn_(b)O_(x).    -   a chemo/electro-active material that comprises        Ta_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x).

Of the above, one or more members of the group consisting of

-   -   a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ga_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Nb_(a)W_(b)O_(x)    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)    -   a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)    -   a chemo/electro-active material that comprises        Ta_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises Ti_(a)Zn_(b)O_(x)        may contain a frit additive.

In the apparatus of this invention, a chemo/electro-active material thatcomprises M¹ _(a)M² _(b)O_(x) may be selected from the group consistingof

-   -   a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises        Cr_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Fe_(a)La_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Fe_(a)La_(b)O_(x), and    -   a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x)    -   a chemo/electro-active material that comprises        Fe_(a)Ni_(b)O_(x), and    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises        Ni_(a)Zn_(b)O_(x), and    -   a chemo/electro-active material that comprises        Sb_(a)Sn_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)La_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises        Ni_(a)Zn_(b)O_(x), and    -   a chemo/electro-active material that comprises        Sb_(a)Sn_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises        Cr_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ni_(a)Zn_(b)O_(x), and    -   a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)    -   a chemo/electro-active material that comprises        Sb_(a)Sn_(b)O_(x), and    -   a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)    -   a chemo/electro-active material that comprises        Ta_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)    -   a chemo/electro-active material that comprises        Cr_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x),        and    -   a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)    -   a chemo/electro-active material that comprises        Cu_(a)Ga_(b)O_(x), and    -   a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x)    -   a chemo/electro-active material that comprises        Cu_(a)La_(b)O_(x), and    -   a chemo/electro-active material that comprises        Fe_(a)La_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)    -   a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)    -   a chemo/electro-active material that comprises Cu_(a)Ga_(b)O_(x)    -   a chemo/electro-active material that comprises        Cu_(a)La_(b)O_(x), and    -   a chemo/electro-active material that comprises        Fe_(a)La_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)    -   a chemo/electro-active material that comprises        Cu_(a)Ga_(b)O_(x), and    -   a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises        Cu_(a)Ga_(b)O_(x),    -   a chemo/electro-active material that comprises        Cu_(a)La_(b)O_(x), and    -   a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)    -   a chemo/electro-active material that comprises        Mn_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)

In the apparatus of this invention, a chemo/electro-active material thatcomprises M1aM2bOx, or a chemo/electro-active material that comprisesM1aM2bM3cOx, may be selected from the group consisting of

-   -   a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Mn_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x), and    -   a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises        Ta_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises Cr_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises        Ta_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises        Ga_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises        Ga_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)    -   a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)    -   a chemo/electro-active material that comprises        Ta_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)    -   a chemo/electro-active material that comprises        Fe_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Ga_(a)Ti_(b)Zn_(c)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ga_(a)Ti_(b)Zn_(c)O_(x), and    -   a chemo/electro-active material that comprises Nb_(a)W_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Cr_(a)Y_(b)O_(x)    -   a chemo/electro-active material that comprises        Cu_(a)Ga_(b)O_(x),    -   a chemo/electro-active material that comprises Cu_(a)La_(b)O_(x)    -   a chemo/electro-active material that comprises Fe_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ga_(a)Ti_(b)Zn_(c)O_(x), and    -   a chemo/electro-active material that comprises Nb_(a)W_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Sr_(b)O_(x), and    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x)

In the apparatus of this invention, a chemo/electro-active material thatcomprises M1Ox, a chemo/electro-active material that comprises M1aM2bOx,or a chemo/electro-active material that comprises M1aM2bM3cOx, may beselected from the group consisting of

-   -   a chemo/electro-active material that comprises        Ga_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ni_(a)Zn_(b)O_(x), and    -   a chemo/electro-active material that comprises SnO₂        or the group consisting of    -   a chemo/electro-active material that comprises        Ga_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)    -   a chemo/electro-active material that comprises SnO₂,    -   a chemo/electro-active material that comprises        Ta_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x), and    -   a chemo/electro-active material that comprises Pr₆O₁₁        or the group consisting of    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Pr₆O₁₁, and    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)    -   a chemo/electro-active material that comprises Mn_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)    -   a chemo/electro-active material that comprises        Nb_(a)Ti_(b)Zn_(c)O_(x)    -   a chemo/electro-active material that comprises Pr₆O₁₁, and    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x).

In the apparatus of this invention, a chemo/electro-active material thatcomprises M1Ox, or a chemo/electro-active material that comprisesM1aM2bOx may be selected from the group consisting of

-   -   a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ni_(a)Zn_(b)O_(x), and    -   a chemo/electro-active material that comprises SnO₂.        or the group consisting of    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)    -   a chemo/electro-active material that comprises SnO₂, and    -   a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)        or the group consisting of    -   a chemo/electro-active material that comprises SnO₂,    -   a chemo/electro-active material that comprises        Ta_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x).        or the group consisting of    -   a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)    -   a chemo/electro-active material that comprises        Sb_(a)Sn_(b)O_(x), and    -   a chemo/electro-active material that comprises ZnO.        or the group consisting of    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)    -   a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)    -   a chemo/electro-active material that comprises        Ta_(a)Ti_(b)O_(x), and    -   a chemo/electro-active material that comprises ZnO        or the group consisting of    -   a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)    -   a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x), and    -   a chemo/electro-active material that comprises ZnO        or the group consisting of    -   a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x), and    -   a chemo/electro-active material that comprises ZnO.        or the group consisting of    -   a chemo/electro-active material that comprises Nb_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises Ni_(a)Zn_(b)O_(x)    -   a chemo/electro-active material that comprises Sb_(a)Sn_(b)O_(x)    -   a chemo/electro-active material that comprises Ta_(a)Ti_(b)O_(x)    -   a chemo/electro-active material that comprises        Ti_(a)Zn_(b)O_(x), and    -   a chemo/electro-active material that comprises ZnO.        or the group consisting of    -   a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises        Cr_(a)Mn_(b)O_(x), and    -   a chemo/electro-active material that comprises CuO        or the group consisting of    -   a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)    -   a chemo/electro-active material that comprises CuO, and    -   a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)        or group consisting of    -   a chemo/electro-active material that comprises CuO    -   a chemo/electro-active material that comprises        Nb_(a)Sr_(b)O_(x), and    -   a chemo/electro-active material that comprises Pr₆O₁₁        or group consisting of    -   a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)    -   a chemo/electro-active material that comprises Pr₆O₁₁, and    -   a chemo/electro-active material that comprises WO₃.        or group consisting of    -   a chemo/electro-active material that comprises Al_(a)Ni_(b)O_(x)    -   a chemo/electro-active material that comprises Cr_(a)Mn_(b)O_(x)    -   a chemo/electro-active material that comprises CuO    -   a chemo/electro-active material that comprises Nb_(a)Sr_(b)O_(x)    -   a chemo/electro-active material that comprises Pr₆O₁₁, and    -   a chemo/electro-active material that comprises WO₃.

Any method of depositing the chemo/electro-active material to asubstrate is suitable. One technique used for deposition is applying asemiconducting material on an alumina substrate on which electrodes arescreen printed. The semiconducting material can be deposited on top ofelectrodes by hand painting semiconducting materials onto the substrate,pipetting materials into wells, thin film deposition, or thick filmprinting techniques. Most techniques are followed by a final firing tosinter the semiconducting materials.

Techniques for screen-printing substrates with the electrodes andchemo/electro-active materials are illustrated in FIGS. 2-3. FIG. 2depicts a method of using interdigitated electrodes overlaid withdielectric material, forming blank wells into which thechemo/electro-active materials can be deposited. FIG. 3 depicts anelectrode screen pattern for an array of 6 materials which is printed onboth sides of the substrate to provide for a 12-material array chip. Twoof the electrodes are in parallel so it holds only 6 unique materials.Counting down from the top of the array shown in FIG. 3, the top twomaterials can only be accessed simultaneously by the split electrodewith which they have shared contact. Below that is the screen patternfor the dielectric material, which is screen printed on top of theelectrodes on both sides of the substrate to prevent the material frombeing fouled by contact with the gas mixture, such as a deposit of sootthat could reduce the sensitivity of a sensor material to a gas or causea short. Below that is the screen pattern for the actual sensormaterials. This is printed in the holes in the dielectric on top of theelectrodes. When more than one material is used in the array, theindividual materials are printed one at a time.

The geometry of a sensor material as fabricated in an array, includingsuch characteristics as its thickness, selection of a compound orcomposition for use as the sensor, and the voltage applied across thearray, can vary depending on the sensitivity required. If desired, theapparatus may be constructed in a size such that it may be passedthrough an opening that is the size of a circle having a diameter of nomore than about 150 mm, or no more than about 100 mm, or no more thanabout 50 mm, or no more than about 25 mm, or no more than about 18 mm,as the requirements of it usage may dictate. The sensor materials arepreferably connected in parallel in a circuit to which a voltage ofabout 1 to about 20, preferably about 1 to about 12, volts is appliedacross the sensor materials.

As noted, the types of electrical response characteristics that may bemeasured include AC impedance or resistance, capacitance, voltage,current or DC resistance. It is preferred to use resistance as theelectric response characteristic of a sensor material that is measuredto perform analysis of a gas mixture and/or a component therein. Forexample, a suitable sensor material may be that which, when at atemperature of about 400° C. or above, has a resistivity of at leastabout 1 ohm-cm, and preferably at least about 10 ohm-cm, and yet no morethan about 10⁶ ohm-cm, preferably no more than about 10⁵ ohm-cm, andmore preferably no more than about 10⁴ ohm-cm. Such a sensor materialmay also be characterized as that which exhibits, preferably at atemperature of about 400° C. or above, upon exposure to a gas mixture, achange in resistance of at least about 0.1 percent, and preferably atleast about 1 percent, as compared to the resistance in the absence ofexposure. Using such material, a signal may be generated that isproportional to the resistance of exhibited by the material when it isexposed to a multi-component gas mixture.

Regardless of the type of response characteristic that is measured forthe purpose of analyzing a mixture and/or a gaseous component ofinterest therein, it is desirable that a sensor material be utilized forwhich a quantified value of that response characteristic is stable overan extended period of time. When the sensor material is exposed to amixture containing the analyte, the concentration of the analyte being afunction of the composition of the particular gas mixture in which it iscontained, the value of the response of the sensor material willpreferably remain constant or vary to only a small extent duringexposure to the mixture over an extended period of time at a constanttemperature. For example, the value of the response, if it varies, willvary by no more than about twenty percent, preferably no more than aboutten percent, more preferably no more than about five percent, and mostpreferably no more than about one percent over a period of at leastabout 1 minute, or preferably a period of hours such as at least about 1hour, preferably at least about 10 hours, more preferably at least about100 hours, and most preferably at least about 1000 hours. One of theadvantages of the types of sensor materials described above is that theyare characterized by this kind of stability of response.

The electrical response characteristic exhibited by achemo/electro-active material in respect of a multi-component gasmixture that contains an analyte gas or sub-group of gases derives fromcontact of the surface of the chemo/electro-active material with the gasmixture containing the analyte(s). The electrical responsecharacteristic is an electrical property, such as capacitance, voltage,current, AC impedance, or AC or DC resistance, that is affected byexposure of the chemo/electro-active material to the multi-component gasmixture. A quantified value of, or a signal proportional to thequantified value of, the electrical property or a change in theelectrical property may be obtained as a useful measurement at one ormore times while the material is exposed to the gas mixture.

An electrical response is determined for each chemo/electro-activematerial upon exposure of the array to a gas mixture, and means fordetermining the response include conductors interconnecting the sensormaterials. The conductors are in turn connected to electrical input andoutput circuitry, including data acquisition and manipulation devices asappropriate to measure and record a response exhibited by a sensormaterial in the form of an electrical signal. The value of a response,such as a measurement related to resistance, may be indicated by thesize of the signal. One or more signals may be generated by an array ofsensors as to each analyte component in the mixture, whether the analyteis one or more individual gases and/or one or more subgroups of gases.

An electrical response is determined for each individualchemo/electro-active material separately from that of each of the otherchemo/electro-active materials. This can be accomplished by accessingeach chemo/electro-active material with an electric currentsequentially, using a multiplexer to provide signals differentiatedbetween one material and another in, for example, the time domain orfrequency domain. It is consequently preferred that nochemo/electro-active material be joined in a series circuit with anyother such material. One electrode, by which a current is passed to achemo/electro-active material, can nevertheless be laid out to havecontact with more than one material. An electrode may have contact withall, or fewer than all, of the chemo/electro-active materials in anarray. For example, if an array has 12 chemo/electro-active materials,an electrode may have contact with each member of a group of 2, 3, 4, 5or 6 (or, optionally, more in each instance) of the chemo/electro-activematerials. The electrode will preferably be laid out to permit anelectrical current to be passed to each member of such group ofchemo/electro-active materials sequentially.

A conductor such as a printed circuit may be used to connect a voltagesource to a sensor material, and, when a voltage is applied across thesensor material, a corresponding current is created through thematerial. Although the voltage may be AC or DC, the magnitude of thevoltage will typically be held constant. The resulting current isproportional to both the applied voltage and the resistance of thesensor material. A response of the material in the form of either thecurrent, voltage or resistance may be determined, and means for doing soinclude commercial analog circuit components such as precisionresistors, filtering capacitors and operational amplifiers (such as aOPA4340). As voltage, current and resistance is each a known function ofthe other two electrical properties, a known quantity for one propertymay be readily converted to that of another.

Resistance may be determined, for example, in connection with thedigitization of an electrical response. Means for digitizing anelectrical response include an analog to digital (A/D) converter, asknown in the art, and may include, for example, electrical componentsand circuitry that involve the operation of a comparator. An electricalresponse in the form of a voltage signal, derived as described above asa result of applying a voltage across a sensor material, is used as aninput to a comparator section (such as a LM339). The other input to thecomparator is driven by a linear ramp produced by charging a capacitorusing a constant current source configured from an operational amplifier(such as a LT1014) and an external transistor (such as a PN2007a). Theramp is controlled and monitored by a microcomputer (such as aT89C51CC01). A second comparator section is also driven by the rampvoltage, but is compared to a precise reference voltage. Themicrocomputer captures the length of time from the start of the ramp tothe activation of the comparators to generate a signal based on thecounted time.

The resistance of the sensor material is then calculated, or quantifiedas a value, by the microcomputer from the ratio of the time signalderived from the voltage output of the material to a time signalcorresponding to a known look-up voltage and, ultimately, to theresistance that is a function of the look-up voltage. A microprocessorchip, such as a T89C51CC01, can be used for this function. Themicroprocessor chip may also serve as means for determining a change inthe resistance of a sensor material by comparing a resistance,determined as above, to a previously determined value of the resistance.

Electrical properties such as impedance or capacitance may bedetermined, for example, by the use of circuitry components such as animpedance meter, a capacitance meter or inductance meter.

Means for digitizing the temperature of an array of chemo/electro-activematerials can include, for example, components as described above thatconvert a signal representative of a physical property, state orcondition of a temperature-measuring device to a signal based on countedtime.

In one embodiment, analysis of a multi-component gas mixture is completeupon the generation of an electrical response, such as resistance, inthe manner described above. As a measurement of resistance exhibited bya sensor material upon exposure to a gas mixture is a function of thepartial pressure within the mixture of one or more component gases, themeasured resistance provides useful information about the composition ofthe gas mixture. The information may, for example, indicate the presenceor absence within the mixture of a particular gas or subgroup of gases.In other embodiments, however, it may be preferred to manipulate, orfurther manipulate, an electrical response in the manner necessary toobtain information related to the concentration within the mixture ofone or more particular component gases or subgroups of gases, or tocalculate the actual concentration within the mixture of one or morecomponent gases or subgroups.

Means for obtaining information concerning the relative concentrationwithin the mixture of one or more individual component gases and/or oneor more subgroups of gases, or for detecting the presence of, orcalculating the actual concentration of, one or more individualcomponent gases and/or subgroups within the mixture, may include amodeling algorithm that incorporates either a PLS (Projection ontoLatent Systems) model, a back-propagation neural network model, or acombination of the two, along with signal pre-processing and outputpost-processing. Signal pre-processing includes, but is not limited to,such operations as principle component analyses, simple lineartransformations and scaling, logarithmic and natural logarithmictransformations, differences of raw signal values (e.g., resistances),and differences of logarithmic values. The algorithm contains a modelwhose parameters have been previously determined, and that empiricallymodels the relationship between the pre-processed input signal andinformation related to the gas concentration of the species of interest.Output post-processing includes, but is not limited to, all of theoperations listed above, as well as their inverse operations.

The model is constructed using equations in which constants,coefficients or other factors are derived from pre-determined valuescharacteristic of a precisely measured electrical response of anindividual sensor material to a particular individual gas or subgroupexpected to be present as a component in the mixture to be analyzed. Theequations may be constructed in any manner that takes temperature intoaccount as a value separate and apart from the electrical responsesexhibited by the sensor materials upon exposure to a gas mixture. Eachindividual sensor material in the array differs from each of the othersensors in its response to at least one of the component gases orsubgroups in the mixture, and these different responses of each of thesensors is determined and used to construct the equations used in themodel.

A change of temperature in the array may be indicated by a change in thequantified value of an electrical response characteristic, resistancefor example, of a sensor material. At a constant partial pressure in themixture of a gas of interest, the value of an electrical responsecharacteristic of a sensor material may vary with a change intemperature of the array, and thus the material. This change in thevalue of an electrical response characteristic may be measured for thepurpose of determining or measuring the extent of change of, and thus avalue for, temperature. The temperature of the array will be the same,or substantially the same, as the temperature of the gas mixture unlessthe array is being maintained at a pre-selected temperature by a heaterlocated on the substrate. If the array is being heated by a heater, thetemperature of the array will lie substantially in the range withinwhich the heater cycles on and off.

It is not required, but is preferred, that the measurement oftemperature be made independently of information related to thecompositional content of a gas mixture. This can be done by not usingsensors that provide compositional information for the additionalpurpose of determining temperature, and, optionally, by connecting thetemperature measuring device in parallel circuitry with the sensormaterials, rather than in series. Means for measuring temperatureinclude a thermocouple or a pyrometer incorporated with an array ofsensors. If the temperature determining device is a thermistor, which istypically a material that is not responsive to an analyte gas, thethermistor is preferably made from a different material than thematerial from which any of the gas sensors is made. Regardless of themethod by which temperature or change in temperature is determined, atemperature value or a quantified change in temperature is a desirableinput, preferably in digitized form, from which an analysis of a mixtureof gases and/or a component therein may be performed.

In the method and apparatus of this invention, unlike various prior-arttechnologies, there is no need to separate the component gases of amixture for purposes of performing an analysis, such as by a membrane orelectrolytic cell. There is also no need when performing an analysis bymeans of this invention to employ a reference gas external to thesystem, such as for the purpose of bringing a response or analyticalresults back to a base line value. A value representative of a referencestate may, however, be used as a factor in an algorithm by whichinformation related to the composition of the gas mixture is determined.With the exception of preliminary testing, during which a standardizedresponse value to be assigned to the exposure of each individual sensormaterial to each individual analyte gas is determined, the sensormaterials are exposed only to the mixture in which an analyte gas and/orsubgroup is contained. The sensor materials are not exposed to any othergas to obtain response values for comparison to those obtained fromexposure to the mixture containing an analyte. The analysis of themixture is therefore performed only from the electrical responsesobtained upon exposure of the chemo/electro-active materials to themixture containing the analyte. No information about an analyte gasand/or subgroup is inferred by exposure of the sensor materials to anygas other than the analyte itself as contained within the mixture.

This invention is therefore useful at the higher temperatures found inautomotive emission systems, typically in the range of from about 400°C. to about 1000° C. In addition to gasoline and diesel internalcombustion engines, however, there is a variety of other combustionprocesses to which this invention could be applied, including stack orburner emissions of all kinds such as resulting from chemicalmanufacturing, electrical generation, waste incineration and airheating. These applications require the detection of gases such asnitrogen oxides, ammonia, carbon monoxide, hydrocarbons and oxygen atthe ppm to per cent levels, typically in a highly corrosive environment.

When the multi-component gas mixture comprises a nitrogen oxide, ahydrocarbon, or both, or any of the other gases mentioned herein, theapparatus may be used to determine the presence and/or concentration ofa nitrogen oxide and/or hydrocarbon in the multi-component gas mixture.The apparatus may also be used to determine the presence and/orconcentration of any one or more to the other gases mentioned hereinthat may be present in a multi-component gas mixture. For this purpose,the electrical response, in the apparatus of this invention, of one ormore of a chemo/electro-active material that comprises M¹O_(x), achemo/electro-active material that comprises M¹ _(a)M² _(b)O_(x), and achemo/electro-active material that comprises M¹ _(a)M² _(b)M³ _(c)O_(x),may be related to one or more of the presence of a nitrogen oxide withinthe gas mixture, the presence of a hydrocarbon within the gas mixture,the collective concentration of all nitrogen oxides within the gasmixture, and the concentration of a hydrocarbon within the gas mixture.

This invention therefore provides methods and apparatus for directlysensing the presence and/or concentration of one or more gases in anmulti-component gas system, comprising an array of at least twochemo/electro-active materials chosen to detect analyte gases orsubgroups of gases in a multi-component gas stream. The multi-componentgas system can be at essentially any temperature that is not so low orso high that the sensor materials are degraded or the sensor apparatusotherwise malfunctions. In one embodiment, the gas system may be at alower temperature such as room temperature (about 25° C.) or elsewherein the range of about 0° C. to less than about 100° C., whereas in otherembodiments the gas mixture may at a higher temperature such as in therange of about 400° C. to about 1000° C. or more. The gas mixture maytherefore have a temperature that is about 0° C. or more, about 100° C.or more, about 200° C. or more, about 300° C. or more, about 400° C. ormore, about 500° C. or more, about 600° C. or more, about 700° C. ormore, or about 800° C. or more, and yet is less than about 1000° C., isless than about 900° C., is less than about 800° C., is less than about700° C., is less than about 600° C., is less than about 500° C., is lessthan about 400° C., is less than about 300° C., is less than about 200°C., or is less than about 100° C.

In applications in which the gas mixture is above about 400° C., thetemperature of the sensor materials and the array may be determinedsubstantially only, and preferably is determined solely, by thetemperature of the gas mixture in which a gaseous analyst is contained.This is typically a variable temperature. When higher-temperature gasesare being analyzed, it may be desirable to provide a heater with thearray to bring the sensor materials quickly to a minimum temperature.Once the analysis has begun, however, the heater (if used) is typicallyswitched off, and no method is provided to maintain the sensor materialsat a preselected temperature. The temperature of the sensor materialsthus rises or falls to the same extent that the temperature of thesurrounding environment does. The temperature of the surroundingenvironment, and thus the sensors and the array, is typically determinedby (or results from) substantially only the temperature of the gasmixture to which the array is exposed.

In applications in which the gas mixture is below about 400° C., it maybe preferred to maintain the sensor materials and the array at apreselected temperature of about 200° C. or above, and preferably 400°C. or above. This preselected temperature may be substantially constant,or preferably is constant. The preselected temperature may also be about500° C. or above, about 600° C. or above, about 700° C. or above, about800° C. or above, about 900° C. or above, or about 1000° C. or above.This may be conveniently done with a heater incorporated with the array,in a manner as known in the art. If desired, a separate micro heatermeans may be supplied for each separate chemo/electro-active material,and any one or more of the materials may be heated to the same or adifferent temperature. The temperature of the gas mixture in such casemay also be below about 300° C., below about 200° C., below about 100°C., or below about 50° C. In these low temperature application, themeans for heating the chemo/electro-active materials may be a voltagesource that has a voltage in the range of about 10⁻³ to about 10⁻⁶volts. The substrate on which the materials are placed may be made of amaterials that is selected from one or more of the group consisting ofsilicon, silicon carbide, silicon nitride, and alumina containing aresistive dopant. Devices used in these low temperature applications areoften small enough to be held in the human hand.

This heating technique is also applicable, however, to the analysis ofhigh temperature gases. When the temperature of the gas mixture is aboveabout 400° C., the sensor materials may nevertheless be maintained by aheater at a constant or substantially constant preselected temperaturethat is higher than the temperature of the gas mixture. Such preselectedtemperature may be about 500° C. or above, about 600° C. or above, about700° C. or above, about 800° C. or above, about 900° C. or above, orabout 1000° C. or above. Should the temperature of the gas mixtureexceed the temperature pre-selected for the heater, the heater may beswitched off during such time. A temperature sensor will still beemployed, however, to measure the temperature of the gas mixture andprovide that value as an input to an algorithm by which informationrelated to the composition of the gas mixture is determined.

In summary, it may be seen that this invention provides means todetermine, measure and record responses exhibited by each of thechemo/electro-active materials present in an array upon exposure to agas mixture. Any means that will determine, measure and record changesin electrical properties can be used, such as a device that is capableof measuring the change in AC impedance of the materials in response tothe concentration of adsorbed gas molecules at their surfaces. Othermeans for determining electrical properties are suitable devices tomeasure, for example, capacitance, voltage, current or DC resistance.Alternatively a change in temperature of the sensing material may bemeasured and recorded. The chemical sensing method and apparatus mayfurther provide means to measure or analyze a mixture and/or thedetected gases such that the presence of the gases are identified and/ortheir concentrations are measured. These means can includeinstrumentation or equipment that is capable, for example, of performingchemometrics, neural networks or other pattern recognition techniques.The chemical sensor apparatus will further comprise a housing for thearray of chemo/electro-active materials, the means for detecting, andmeans for analyzing.

The device includes a substrate, an array of at least twochemo/electro-active materials chosen to detect one or morepredetermined gases in a multi-component gas stream, and a means todetect changes in electrical properties in each of thechemo/electro-active materials present upon exposure to the gas system.The array of sensor materials should be able to detect an analyte ofinterest despite competing reactions caused by the presence of theseveral other components of a multi-component mixture. For this purpose,this invention uses an array or multiplicity of sensor materials, asdescribed herein, each of which has a different sensitivity for at leastone of the gas components of the mixture to be detected. A sensor thathas the needed sensitivity, and that can operate to generate the typesof analytical measurements and results described above, is obtained byselection of appropriate compositions of materials from which the sensoris made. Various suitable types of materials for this purpose aredescribed above. The number of sensors in the array is typically greaterthan or equal to the number of individual gas components to be analyzedin the mixture.

Further description relevant to the apparatus of this invention, usesfor the apparatus and methods of using the apparatus may be found inU.S. Provisional Application No. 60/370,445, filed Apr. 5, 2002, andU.S. application Ser. No. 10/117,472, filed Apr. 5, 2002, each of whichis incorporated in its entirety as a part hereof for all purposes.

1. An apparatus for reducing a nitrogen oxide gas emitted by a emissionssource, comprising (a) an exhaust conduit for transporting the nitrogenoxide gas downstream from the emissions source, (b) an injector forinjecting a reducing agent into the conduit, and (c) one or more gasanalyzers located in the conduit upstream from the injector.
 2. Anapparatus according to claim 1 further comprising a catalyst to catalyzethe reduction of the nitrogen oxide.
 3. An apparatus according to claim2 further comprising a gas analyzer that is downstream from the injectorand downstream from a catalyst.
 4. An apparatus according to claim 2wherein a first gas analyzer is located upstream from a catalyst, and asecond gas analyzer is located downstream from the catalyst.
 5. Anapparatus according to claim 2 that comprises a plurality of gasanalyzers, wherein a plurality of gas analyzers is located upstream froma catalyst, and a plurality of gas analyzers is located downstream fromthe catalyst.
 6. An apparatus according to claim 2 comprising (a) afirst catalyst, (b) a gas analyzer located downstream from the firstcatalyst, and (c) a second catalyst located downstream from the gasanalyzer.
 7. An apparatus according to claim 6 wherein the first andsecond catalysts is each a catalyst bed in vertical arrangement.
 8. Anapparatus according to claim 6 further comprising a plurality of gasanalyzers located between the first and second catalysts.
 9. Anapparatus according to claim 6 further comprising one or more gasanalyzers downstream from the second catalyst.
 10. An apparatusaccording to claim 2 further comprising one or more gas analyzersdownstream from all catalysts.
 11. An apparatus according to claim 1 or2 wherein the reducing agent is ammonia.
 12. An apparatus according toclaim 1 or 2 wherein the reducing agent is urea.
 13. An apparatusaccording to claim 1 or 2 wherein the emissions source is stationary.14. An electrical generating plant comprising an apparatus for reducinga nitrogen oxide gas according to claim 1 or
 2. 15. A furnace comprisingan apparatus for reducing a nitrogen oxide gas according to claim 1 or2.
 16. A steam turbine comprising an apparatus for reducing a nitrogenoxide gas according to claim 1 or
 2. 17. A gas turbine comprising anapparatus for reducing a nitrogen oxide gas according to claim 1 or 2.18. A vehicle for transportation or recreation comprising an apparatusfor reducing a nitrogen oxide gas according to claim 1 or
 2. 19. A pieceof equipment for construction, maintenance or industrial operationscomprising an apparatus for reducing a nitrogen oxide gas according toclaim 1 or
 2. 20. In a multi-component gas mixture that is emitted by aemissions source and contains a nitrogen oxide, wherein a nitrogen oxideis reduced by injecting a reducing agent into the gas mixture, a methodof decreasing the amount or release of unreacted reducing agentcomprising determining information as to the compositional content ofthe gas mixture, and controlling the injection of the reducing agent inrelation to the information as to the compositional content of the gasmixture.
 21. A method according to claim 20 wherein the gas mixture iscontacted with a catalyst, and information as to the compositionalcontent of the gas mixture is determined before the gas mixture contactsany catalyst.
 22. A method according to claim 21 further comprising astep of determining information as to the compositional content of thegas mixture after the gas mixture contacts any catalyst.
 23. A methodaccording to claim 20 wherein the gas mixture is contacted with acatalyst, and information as to the compositional content of the gasmixture is determined after the gas mixture contacts any catalyst.
 24. Amethod according to claim 20 wherein the gas mixture is contacted withfirst and second catalysts, and information as to the compositionalcontent of the gas mixture is determined after the gas mixture contactsa first catalyst but before the gas mixture contacts a second catalyst.25. A method according to claim 20 wherein the gas mixture is contactedwith a catalyst, and information as to the compositional content of thegas mixture is determined after the gas mixture contacts all catalyst.26. A method according to claim 20, 21, 23 or 25 further comprising astep of determining the amount of reducing agent to be injected into thegas mixture in relation to the information as to the compositionalcontent of the gas mixture.
 27. A method according to claim 20, 21, 23or 25 wherein the information as to the compositional content of the gasmixture is an output of one or more gas analyzers.
 28. A methodaccording to claim 27 wherein the gas mixture is transported downstreamfrom the emissions source by an exhaust conduit, and a gas analyzerlocated in the conduit.
 29. A method according to claim 20, 21, 23 or 25wherein the information as to the compositional content of the gasmixture is determined from an array of chemo/electro-active materials.30. A method according to claim 20, 21, 23 or 25 wherein the informationas to the compositional content of the gas mixture is related to theindividual concentration within the gas mixture of an individual gascomponent therein.
 31. A method according to claim 20, 21, 23 or 25wherein the information as to the compositional content of the gasmixture is related to the collective concentration within the gasmixture of a subgroup of the component gases therein.
 32. A methodaccording to claim 20, 21, 23 or 25 wherein the information as to thecompositional content of the gas mixture is related to the individualconcentration within the gas mixture of an individual gas componenttherein, and is related to the collective concentration within the gasmixture of a subgroup of the component gases therein.
 33. A methodaccording to claim 20, 21, 23 or 25 wherein the information as to thecompositional content of the gas mixture is inputted to adecision-making routine.
 34. A method according to claim 20, 21, 23 or25 wherein the information as to the compositional content of the gasmixture is inputted to a map.
 35. A method according to claim 20, 21, 23or 25 wherein the information as to the compositional content of the gasmixture is related to the individual concentration within the gasmixture of an individual nitrogen oxide component therein.
 36. A methodaccording to claim 20, 21, 23 or 25 wherein the information as to thecompositional content of the gas mixture is related to the collectiveconcentration within the gas mixture of all nitrogen oxide componentstherein.
 37. A method according to claim 27 wherein a gas analyzercomprises an array of chemo/electro-active materials.
 38. A methodaccording to claim 27 wherein a gas analyzer outputs at least one signalthat is related to the individual concentration within the gas mixtureof an individual gas component therein.
 39. A method according to claim27 wherein a gas analyzer outputs at least one signal that is related tothe collective concentration within the gas mixture of a subgroup of thecomponent gases therein.
 40. A method according to claim 27 wherein agas analyzer outputs at least one signal that is related to theindividual concentration within the gas mixture of an individual gascomponent therein, and at least one signal that is related to thecollective concentration within the gas mixture of a subgroup of thecomponent gases therein.
 41. A method according to claim 27 wherein agas analyzer outputs a signal to a decision-making routine.
 42. A methodaccording to claim 27 wherein a gas analyzer that is upstream from allcatalyst, and a gas analyzer that is downstream from all catalyst, bothoutput a signal to a decision-making routine.
 43. A method according toclaim 27 wherein a gas analyzer that is upstream from all catalyst, agas analyzer that is downstream from a first catalyst and upstream froma second catalyst, and a gas analyzer that is downstream from allcatalyst, each outputs a signal to a decision-making routine.
 44. Amethod according to claim 27 wherein the gas analyzer outputs a signalto a map.
 45. A method according to claim 27 wherein a gas analyzeroutputs a signal to a decision-making routine that controls theinjection of reducing agent.
 46. A method according to claim 27 whereina gas analyzer outputs a signal to a decision-making routine thatcalculates an amount of reducing agent to be injected.
 47. A methodaccording to claim 27 wherein the gas analyzer outputs at least onesignal that is related to the individual concentration within the gasmixture of an individual nitrogen oxide component therein.
 48. A methodaccording to claim 27 wherein the gas analyzer outputs at least onesignal that is related to the collective concentration within the gasmixture of all nitrogen oxide components therein.
 49. A method accordingto claim 27 wherein a gas analyzer outputs at least one signal that isrelated to the individual concentration within the gas mixture of one ormore or all of the nitrogen oxide component(s) therein, and the signalis outputted to a decision-making routine that calculates an amount ofreducing agent to be injected.
 50. A method according to claim 20, 21,23 or 25 wherein the emissions source is stationary.
 51. A methodaccording to claim 20, 21, 23 or 25 wherein the emissions source is avehicle for transportation or recreation or a piece of equipment forconstruction, maintenance or industrial operations.